The final third of the Spellcraft & Swordplay core rules. For those who want the three stapled little brown booklet feel. From the publisher:

Elf Lair Games presents Monsters & Magic, volume 3 in the Spellcraft & Swordplay Old School Edition roleplaying game. For the Referee's eyes only, this book contains statistics for dozens of monsters and magic items, and guidelines for awarding experience and even creating your own critters! It features all errata from the first and second printings incorporated, revised rules that remove holdovers from older games, all still in that slick old-school format! Give your game a REAL old school shot in the arm with the REVISED OLD SCHOOL EDITION!