From the author's website:
"Magical Theorems & Dark Pacts is a 159-page book of material for Labyrinth Lord and classic 1981-era OSR spellcasters.
At heart, Magical Theorems & Dark Pacts is my revised rule book for Labyrinth Lord spellcasters. Emphasis on the word my. It reprints the Magic-User and Cleric (well, with a few minor tweaks that are hardly worth mentioning except the revised clerical turning undead table), and then piles on a bunch of other spellcasting classes along with their spell lists and spells. I also end up renaming the Magic-User as the Wizard because I use the term Magic-User as an umbrella for all arcane spellcasting classes.