TSR was by mid 1977 really on a roll with D&D, accessories for the game were starting to come out faster and the variety was increasing. After a long time with just the original booklets and some supplements, we are getting a Magazine regularly as well as stuff like the Geomorphs and now this. 

This is a 16 page booklet, the first of three, that as the title says consists of basically lists of Monsters and Treasure 100 of each for each level, as a way to populate your dungeons and make them more interesting while saving the DM a lot of trouble. Here you roll dice to determine traps and containers for treasure and then percentile die to determine monsters and content. There’s a page of instructions and some cute illustrations. Pretty useful stuff for the kind of sandbox dungeons that were all the rage at the time.