The Book of Elder Magic by Daniel Boggs.
If you're into the original game or WBFMAG or any other ODnD clone, the Book of Elder Magic is certainly something you should look at.
It's an expanded spell book for Clerics and Magic-Users based on the author's extensive research into OD&D.... 0ed ie how DnD might have turned out if Gary Gygax hadn't been in such a hurry to publish and had added in Dave Arneson's ideas and play test corrections instead. The full set of rules Daniel has come up with is called Champions of ZED (zero edition) and also the game engine which powers the Lost Dungeons of Tanisborg.
This book consists of all of the spells from ODnD plus the extra ones gleaned from Arneson's notes and other early play testers. They are completely compatible with ODnD, Champions of ZED, and in fact, any other clone based on ODnD, B/X, Holmes or BECMI.
The softback is about $8 on Drivethru.