The Dungeons & Dragons Rules Cyclopedia in PDF form has just been re-released, and although that version (as well as BECMI) was published after my introduction to D&D (via Basic/Expert D&D and AD&D), I know there are others for whom it was the definitive edition. It is currently #1 in the list of "Hottest Items" at dndclassics.com. (I am happy to report that the D&D Basic Rulebook and Expert Rulebook of 1981 are still in the top five.)
If only Basic/Expert could have been integrated into one volume...