Darth Plagious really is your best bet if you just want anything. It does offer the most conclusive information about his rise. Nothing else really covers it to the same extent. From the current canon Lords of the Sith might contain some information but honestly I do not know as I have not gotten around to reading it yet. The Darth Vader comics actually have some pretty sick insight into the fact that Palpatine cared little of preserving the Sith way. In those you find that he is actually having a group of cyberneticly enhanced people trained to essentially replace Darth Vader some day. He is more concerned with usefulness than any Sith ideal, which I would say is a fairly stark contrast to Legends information (where yes, he didn't care about the rule of two so much but he did care about Sith stuff).
Okay his question is lore oriented, not solely RP related from the post. As a star wars fan, I still do canon and am concerned about canon. Perhaps that makes me an oddball here given that I don't desire to bend canon to my desires when I RP/RP'd. Perhaps the information about the backgrounds of these characters is different in the present canon vs. legends. It could change how the person who is RPing is influenced.