the Dnd Rules cyclopedia is without a doubt in my mind one of the greatest if not the greatest RPG books of all time. this book is everything a person needs to play Basic dungeons and dragons from levels 1-36 (why 36..who knows). now a lot of books say or claim to be everything you need, but with this book they are not kidding. not only does it have the basic character building and monsters, but it has rules on running kingdoms, mass combat, becoming immortals, even a primer on a campaign setting that has maps and a short wright up of several kingdoms in that setting.
now this book does have a few flaws. one is the lack of art work, now the art that is in the book is very good, but it does have little art work due to cramming so much into the book. the other issue for newer modern gamer’s is the system uses the older dnd system of the pre 3rd edition era which can be off putting to some. now i think the system is fine and is a lot smother then the Ad&d system, but that is a personal preference. one other issue for some is that race and classes are not separate for the most part. for example elf is your race and class (in this case a fighter/mage mix). humans on the other hand can pick a class (fighter, mage, cleric, thief, mystic) the races are elf, dwarf, and halfling.
one of my favorite rules in the book is that of weapon mastery. theses rules can turn a simple fighter into a walking death machine. there is no linear fighter quadratic wizard here folks when weapon mastery is used. a grand master of a weapon can one shot half the monsters in the book and could take a wizard down before a single spell is cast.
now for people who are use to other none basic editions of dnd I would recommend reading the book through as there are minor albeit important differences with spells and a how attacks are calculated. one example is magic missile a classic first level spell. in 2e and beyond it is a few instant energy bolts that shoot at an opponent. in this version several magical arrows appear around you and can last up to a min (i think a min) that you can choose to fire when you need them to. they also do more damage then later versions of Magic Missile (1D6 vs 1D4+1 per missile)
overall if you are a collector or want a truly self contained old school rpg i suggest finding a copy, they are pricey though. this price is not due to rarity, but do to no one wanting to sell their copies due to how awesome the book is.