From the back of the book:
When you've reached as far as you possibly can there's only one thing to do; extend your reach with the DUNGEONS & DRAGONS Expert Rules.
The rules within this booklet are additions and expansions to the D&D game system. Characters may now reach for new heights of power and greatness. This booklet details the expansion of characters up to 14th level. Within are new spells, magic items and over 100 new monsters, all of which will add new excitement to any D&D game.
The rule booklet is easy to read and understand, and the rules have been organized into eight modular sections that can easily be integrated with the D&D Basic Rules.
This rulebook was sold separately, as well as being part of the Dungeons & Dragons Expert Set. The PDF version of this rulebook is also available and includes a scan of the 1981 Gateway to Adventure TSR Catalog.