From publisher blurb:
Bring additional dimensions to your classic fantasy gaming! This 64-page book presents players and DMs alike with information to expand the playability and possibilities in their campaigns. It's a cornucopia of information on all aspects of the game including characters, spellcasting, adventures, encounters, monsters and dungeon mastering. This is truly an indispensable resource for the the classic edition Dungeon Master (BX, OSE, LL).
detailed and expanded Ability Score information
character background generation
dual-axis alignment information, including alignments for all standard B/X monsters
detailed language information
new cleric options for petitions, manufacture of holy water, and turning based on the Hit Dice of the undead creature
new options for demi-humans establishing their own clans, including the acquisition and use of clan artifacts
new combat options for fighters
new magic-user options for cantrips, point-based spellcasting, manufacture of spell scrolls, and familiars
new thief options for finding, disarming, and setting traps
a system for creating custom character classes
expanded weapon and armor information
full rules for hiring and using henchmen, hirelings, mercenaries, and specialists
comprehensive alchemy rules, including descriptions for 64 different potions, and potion miscibility information
information about animal training
new rules for lore checks, reactions, object saving throws, unarmed combat, invisibility, morale, sanity & madness, the effects of alcohol & drugs, and poison use
expanded information to assist with monster encounters, and rules for monsters as tribal spellcasters
information about adventuring on the various Planes of Existence
details about creation and campaign use of artifacts