Alternative Title: "Crouching Dungeon, Hidden Dragons"
Alright, so I said I'd need to do this one eventually, and it's been a while since I've done one of these posts. This is going to be a really long one. Prepare for tl;dr bait.

I've mentioned in previous posts that there were some major issues with 3.5e D&D, mostly in the realm of game balance. Main casters, who gained anywhere from one to three new abilities usable per day every level, simply -wildly- outstripped the basic "hit something with a stick" classes by fifth level. The classes were simply designed in entirely different ways, which makes power comparisons between them impossible.
Put simply, warrior classes never learn any new tricks, while caster classes learn nothing -but- new tricks. There can be no competition. A monk, for instance, learns an ability that mimics a spell and can be used once per day. Sure, it also gets attack bonuses (which are very underwhelming) and defense bonuses (which are okay at best), but the separation between the power level of an activated ability with limited uses and a passive ability that's always on is pretty vast. When wizards start getting spells that have hours of duration, they function as passives anyway, leaving warriors' heaps of passive abilities in the dust. As designed, casters will always be superior to warriors in any given situation.
Why am I harping on this? Because ToB fixes ALL OF THAT.
Holy Crap This Book Is Awesome
"Tome of Battle: The Book of Nine Swords" is a 3.5e splatbook which singlehandedly closes the gap between warrior classes and mage classes. Contained within is a whole book of "martial maneuvers" and "stances" for warrior characters to use, giving them the power of spells without the limiting factors of uses per day. These range from hitting harder to hitting more things, from disappearing from sight to throwing someone across the field, from channeling divine might to just reacting faster. It's versatile, it's powerful, and it makes playing a warrior much less of a headache when among a bunch of mages.
The book introduces three new classes: the Crusader, a paladin-type with a much less restricted alignment; the Swordsage, an esoteric monk/ninja/spellsword who learns some pretty much overtly magical stuff; and the Warblade, a simple warrior who might use rage or skill to beat people into submission. It's got tons of prestige classes, too, but most of them are either totally useless or completely overpowered (as is normal for 3.5e PrC's). A very healthy number of new feats fits in with the new content spectacularly, and continues to add to the feeling of being a thematic warrior instead of a list of numbers on a sheet.
Also, there's a feat which mimics a weaker version of the monk's unarmed damage, in case you want to play a martial artist that doesn't suck.
The meat of the book is devoted to the eponymous Nine Swords, nine styles of combat that each give a character a different feel, and can be mixed and matched with surprising freedom. Each of the three new classes has access to a certain number of them, and each class has at least one unique form inaccessible to the other two (Swordsage has three!) to give each a distinct flavor. And yes, if it feels like I'm describing wizard schools of magic, there's a reason for that.
Each individual style is populated by maneuvers and stances. A maneuver is an activated ability, either a boost (which augments your abilities), a strike (which replaces your basic attack), or a counter (not unlike an attack of opportunity). All three can be used once per encounter, like a minor spell which replaces your action, but each of the classes in the book has a recharge mechanic that allows them to get more use out of their abilities in long battles. A stance is a passive ability which is constantly in effect until you switch to another stance, and can be used whenever.
Maneuvers, not unlike spells, are divided into nine levels, and a new level becomes available every odd-numbered level in a class that grants maneuvers. However, -unlike- spellcasting, you still gain skill with martial maneuvers even when you're taking levels in another class. Half of your non-maneuver class levels are counted for your effective "initiator level." A character who took one level of warblade then six levels of fighter would functionally count as having another three "levels" of warblade for the purpose of what abilities she could learn if she took another warblade level. This makes multiclassing MUCH easier for ToB material than it is for PHB casters, and that's a very good thing, considering how fighter is probably the best "dip" class in the game.
There's even a progression system in place, so not only will you get stronger over time, but you have to invest effort into learning the fundamentals of a style before you can pick up the really sweet endgame techniques. If you want to use the extremely powerful Time Stands Still maneuver, for instance, you need to have 17 effective martial levels under your belt, and know four other, weaker maneuvers from the Diamond Mind style.
The Fluff
Ooooooh buddy. If you're into plot, this book is like fantasy geek heroin. The background on this world of unique martial combat is thick and gorgeous. The Sublime Way is touted like kung fu, and the whole thing has a very wuxia feel to it. House of Flying Daggers meets Lord of the Rings in a guaranteed Anna fluff-gasm experience. Hobgoblin weaponmasters, halfling monks, Kung Fu Jesus and his nine disciples, betrayal and carnage...everything you could possibly want to base your whole setting around or just spice up an existing world is right there in the pages.
At the beginning of each chapter, we're treated to a stylized painted image of Kung Fu Jesus, better known as Reshar, as he journeys through the world to learn and master all nine steps along the Sublime Way. We get brief insights into his adventures, small teasers that give us a taste of the flavor of the world without railroading a hard plot for Reshar's life.
The plot of the Temple of the Nine Swords is outlined in the pre-chapter memoires of a swordsage named Harran as he recounts his adventures and explains the Sublime Way, all coated in flavor. The full story gets the last few chapters of the book, which describes the actual, physical Nine Swords, the temple, and the tragedy that struck it in the face of greed and betrayal.
Furthermore, as hit-or-miss as the prestige classes are mechanically, thematically they're utterly gorgeous. Eternal Blade elven swordsmen who are guided by the ancient spirits of their own warrior ancestors. Jade Phoenix Mages, unable to find the peace of death as they guard the world from a monstrosity they unleashed. Dwarven weaponmasters of a timeless tradition who master the Stone Dragon style taught to them from the very living rock. The Masters of Nine who seek to emulate Kung Fu Jesus himself. Sure, not all of them are functional, but many are an absolute pleasure to read.
But I'm Not One To Ignore Flaws
As much as I freaking adore this splat, it has some issues. Nothing serious or glaring, of course, but it's by no means perfect. The most glaring issue is that this does not fix fighters, monks, rangers, or paladins. It fixes -warrior classes-, by replacing the existing ones with new, better, more versatile and powerful upgrades. All of the PHB warrior classes (with the notable and respected exception of barbarian) continue to suck ass, and this book will do very little to fix that. Warblades and swordsages are almost literally a straight upgrade to fighters and monks, doing everything those classes can do only better. I can understand that, though. The way warrior classes were designed in the PHB is a hurdle that no amount of new feats could possibly hope to fix.
Another issue which is an obstacle for many is the flavor. It's been called the Book of Weeaboo Fightan Magic for a reason. Several people see less wuxia and more anime, and not everyone can tolerate that in their view of what D&D is supposed to be. If you feel like you don't want to play with magic ninja swordfighters using esoteric techniques, try to stay away from swordsage-related stuff.
From my point of view, the weakest point of the book is how shoddy the playtesting and editing seem to have been. The released book includes no less than four high-level maneuvers (7th or higher) with no prerequisites at all, including one capstone. Literally anyone with 17 levels of a martial class can learn the Stone Dragon capstone, even if they've never touched the style before. About half of the PrCs are either niche or trash, too, which is depressing because of the aforementioned thick story laid into them.
I also would've liked to see a lot more monsters. The three or four they gave us really won't help a long-running game fill out their rogue's gallery. The world itself is just detailed enough to allow you to play around in it, but not so detailed that you can base an entire campaign around it. The monsters aren't even that. Lovely concepts, decent execution, but such a slim repertoire.
TL;DR Version
This is my favorite splatbook in the history of D&D. It was the gateway to the 4th Edition that I love so much, it influenced the way fighters and monks were designed in 5th Edition, and even in 3.5e it substantially levels the playing field between people who want to play valiant knights and those who want to play all-powerful mages. The Three new classes are fantastic, the maneuvers are exactly what warriors needed, the feats are comprehensive, and the story is fabulous.
This book is incredible. Anyone bored playing a whack-bot fighter or just craving more party balance should check it out ASAP.
Post-Script: Anna's Favorites
I did a blog post about Shriastae earlier. -This- is the book I use to build her. For a spellsword-minded gamer like me, the Jade Phoenix Mage prestige class is a chocolate mousse cake with caramel and raspberry toppings. It has everything you could possibly want, with high casing progression (8/10), full BAB, and some special tricks to make casting in combat easier. It even includes a class feature version of my favorite 3.5e feat, Arcane Strike, and a gloriously thematic capstone allowing its user to be reborn in fire.
And oh god the plot have I mentioned the -plot?!- The Jade Phoenix mages are an order of ancient peacekeepers who initially followed the Sublime Way for power, until their hubris and ambition brought an unthinkable evil to the world. The entire order stood and fought this darkness until most were slaughtered and the beast was contained. Their sheer conviction and arcane power force their spirits to reincarnate until the end of time, so their watch over the monster at the end of the world will never waver.
For styles, it's really hard to pick, but I think Devoted Spirit has the most potential. It can be built for tanking, the best and only reliable combat healing in the game, or both. The feat "Superior Unarmed Strike" is also probably the best standalone feat in the book, though Martial Study allows for the most customization. I am a total sucker for options, and three extra maneuvers for anyone in any class? Hell yes.