B/X Essentials: Core Rules

From publisher blurb:

The B/X Essentials line is a restatement of the classic Basic/Expert fantasy adventure gaming rule set, distilled down to its purest essence and given a fresh, new presentation:

Basic and Expert rules seamlessly combined.
Streamlined presentation optimised for ease of reference during play.
Meticulously researched; a guaranteed 100% accurate rendition of the classic rules of yesteryear.
Carefully clarified, ironing out ambiguities in the original rules.

This first book lays out the core rules of the game, including: encounters, combat, dungeon and wilderness exploration, seafaring, spell casting, and magical research.