I went looking to buy a used copy of the Rules Cyclopedia and found the price prohibitive. A little reading online sent me to check a few retro-clones, and this one seems to be the nearest thing to the original. It has some minor modifications, which for the most part are improvements or errata corrections. I ended up also getting a print copy from Lulu and it is very reasonably priced - not sure if it is print on demand from this site. If you are looking for some retro D&D style play this is a great all in one rule book, well worth the cost. Digital copy was perfectly clear.
[5 of 5 Stars!]