I'm pretty minimalist on my non-core books, myself.
I like to keep things tight & simple. I like to have one to three core books and then mostly disregard everything else. If the core rules are done right, that serves as a springboard to everything else I want to do.
Arcana Evolved is a perfect example - it has characters, spells, setting, monsters, and rules all put together.
If I'm using a premade setting, I want
one book or boxed set. Past that, I won't buy it.
I will not use treasuries of spells, prestige classes, feats, or magic items. I don't like having to potentially retool everything, and I don't like scampering around for references.
I
would use a spell treasury if it included all the core spells with it. I know this would never be done, but damn would it be nice.
I can and do use the hell out of monster supplements. Monsters are simple to use piecemeal.
In short, I want to reference no more than two books at the gaming table, not including monster books. I consider the PHB+DMG as one book.
This means I'm a lousy RPG customer.
-O