I have to admit, I loved the book, but Sidious didn't track for me - he was so careful and had very clear ideas about what was going to happen next, and had his own little plan whipped up. As you say, though, his betrayal of Plagieus was just...well, we knew it would happen, but there was just no build-up to it. Part of me thinks that was intentional: the way they'd written Plagieus suggests that, if we knew, he would, too, because he was a master manipulator and capable of spotting a plot in motion. Even so, the end was a bit sudden and, honestly, a bit disappointing.
That noted, it's rare we ever see a book from the Sith side that isn't "OMG, POWER!", or the idiocy of Darth Bane (which is similar), all based on a very brutal, badly-thought out philosophy which then plagued Sith history from there-on out. Plagieus brought a touch of sense to the role, even though he endorsed the Rule of Two, but he at least had the sense to see it as a tool, rather than the be-all and end-all. More to the point, the part I find interesting was this: he was powerful in his own right. He had money, influence and considerable power, all of which came from him, not from the fact that he was Sith. That part was just icing.
