I'm a huge fan of Monte Cook's Book of Eldritch Might series, which was collected in the Complete Book of Eldritch Might. I used to hand out the first two as actual books of lore to the PCs: They would find a copy of it in the game world and I would hand over a physical copy.
FFG had a series of supplements -- School of Illusion, School of Evocation, etc. -- that was a little hit-and-miss, but which I've also pulled some really good stuff out of over the years. This includes Spells & Spellcraft. (Disclaimer: I worked as a freelancer for FFG, but not on any of their spell-based products.)
Checking my notes, it looks like I've also pulled substantial chunks out of Bastion Press' Spells & Magic, but it's been long enough since I read the whole thing that I'm not sure how consistent the content is. (Looks like I may have been tweaking spell levels as I pulled them, too, which isn't always a great sign.)
There's also Relics & Rituals and Relics & Rituals 2 from the Swords & Sorcery imprint of White Wolf. I found these to be very inconsistent, but there's enough content there that I frequently dip in and find stuff worth salvaging.