Is it savegame compatible or I need to start a new game?
Mods that don't corrupt the Main folder (ones that don't prevent multiplayer) are inherently incompatible. The way around this (getting it to work with your I progress saves) involves meta.lsb swapping of save games as I described in my other thread Main Campaign Modding.
This mod should be making the originally unsellable Skillbooks sell like any other Skillbook. Which means they will populate a vendors inventory for the first time you talk to a vendor each level (note the minimum level to even be able to see it though).
Was Dust Devil a rare book? It's possible I missed one, since the Links file I looked through is cluttered. I'll look into that later when I'm able to.
Edit:
For your edited questions-
I actually don't know how mods work with multiplayer. I presume all players need it (those are the anecdotes I've been hearing).
All other Skillbooks not touched by this mod are as purchasable as they have always been. Vendors will eventually sell any book within their respective schools; you just need the appropriate level to see the higher level books.
Edit #2:
If your current playthrough uses another mod, then all you would need to do is move the PurchasableSkillbooks.txt from my mods Public/(Mod Name)/Stats/Generated/Links/ folder to the corresponding Links folder for the mod you're using.
