I have the book by Paletta that Rendos mentioned. It is great when you are at the "I don't even know what questions to ask" stage. It steps through the basics very clearly. Also briefly presents most major families of aquarium fish. It does not discuss corals -- it is intended to help you set up a FOWLR (Fish Only With Live Rock) tank. However, you can get this book, and supplement it with another book about corals when you get to that stage. Great book for the price.
I also have "The Coral Reef Aquarium" by Ron Shimek, which is also a good value ($13 at local book store). It's not very long, but I found it useful for describing the basics of a reef aquarium ecosystem, and for another take on setting up a tank. (Shimek's basic setup uses a sump, unlike Paletta's). Shimek also gives a quick species overview of inverts, fish, and algaes.
Two very cheap paperback by John Tullock -- "Corals" and "Clownfishes and Sea Anemones" ($7 each) were useful to familiarize myself with particular species of corals, and to help me put some thought into what type of clown I wanted to get (and what type of anemone, if any, to go with it).
When you consider the amount of money that goes into the tank, $50 for a book is not very much, and it is money well spent. Still, it's hard to drop that kind of cash on a single book when you are getting started. The above books helped me get my feet wet, along with the great advice on particulars offered here.
When I have seen threads about great aquarium books, the Sprung and Delbeek books always are mentioned, along with "The Conscientious Marine Aquarist" (I think it is titled). Those will be my next purchase.