There are a number of factors... obviously how strong the champion is and how high priority, but also what benefits you get from the books (cooldown reductions / higher chance to apply debuffs, or only damage boosts?) and how badly you need those benefits. If we're talking somebody responsible to keep a certain debuff on the Clan Boss, then yeah, you absolutely need 100 percent chance to apply and the shortest possible cooldown. Whereas if it's more an arena champion and most battles will be over after a turn or two anyway, maybe you don't really need that cooldown reduction so badly. Some champions are generally considered to not need books at all, others really do need books, with others it depends on how you use them (e.g. Scyl of the Drakes doesn't need books if you use her primarily as a healer / dungeon carry, but does need books if you use her primarily for that AOE stun).
And then how many books do you have to (potentially) spend before getting to the improvements you actually want? Some champions require 15+ books to upgrade all skills - and it's generally the last ones that you really want, so you have to waste a lot of books to get that far. Due to that, you'll generally want to go all-in on books on some champions and not spend any at all on others, rather than spreading them around a bit.
In your case, both Sicia and Fu-Shan could benefit from books, it wouldn't be crazy to invest them in either one, but they may not be absolutely required depending on what other champions you have and how heavily you rely on their debuffs.