I've been pondering recently, what supplements/manuals are, in your experience, advised/good idea/not particularly problematic when used in conjunction with Pathfinder?
Personally, while I haven't been using too many supplements since I started playing with Pathfinder last year (when my group made the switch, we decided it was time to "go back to the roots" and try to play only with the core rulebooks for a while. However, supplements have been slowly creeping into the game ever since, which is perfectly fine as I enjoy using them. But I disgress).
Of the ones I've been using recently, the one that's getting the most juice extracted without encountering real issues is the Complete Book of Eldritch Might, by Malhavoc Press, as well as the Libris Mortis from Wizards (though since the later has been mostly as a source of monsters, there has been some tinkering around with numbers. Nothing too ghastly, though... see what I did there? Nevermind).
The Spell Compendium and Necromancer's Eldritch Sorcery are two favorites of mine. AEG's Ultimate Toolbox is always helpful (shameless plug). APG is an instant shoe in as our group is using the beta.