The D&D Rules Cyclopedia is now available in pdf from DrivethruRPG/RPGNow/DNDClassics for US$9.95. This is the compilation of the rules from the D&D Basic, Expert, Companion, and Masters boxed sets by Frank Mentzer (from the 1980s) into a single handback set of rules by Aaron Allston. It is probably the best one-volume version of D&D ever written. The artwork leaves something to be desired compared to the art in the original boxed sets, but it isn’t terrible. The rules themselves are very good, although you will want to download a free copy of the fan-produced errata from here.
The D&D Rules Cyclopedia should not be confused with the rules compilation volumes for 3.x and 4e. The D&D Rules Cyclopedia is for BECMI D&D from the 1980s and includes all the information you need to play: the rules, monsters, treasure, GM advice, etc. Everything one needs to play except dice and adventures. It is a bargain at $9.95 even in pdf form. My only regret about this release is that this pdf release probably means WOTC in not planning to reprint this book as they have reprinted the core books for 1e, 2e, and 3e (and will be doing for 0e later this year). The only reason I can figure (read: guess) for not reprinting the D&D Rules Cycopedia is that WOTC is afraid that having a simple to learn and play — but very complete — version of D&D available in hardback would hurt D&D Next sales.
If you do not own a copy of the D&D Rules Cyclopedia, you can get one in PDF from RPGNow here for only $9.95. IMHO, it’s the best $10 anyone can spend on D&D. Your mileage may vary, of course.