Organic Pixels Graphic Design Store
They aren't kidding with the title either. This huge book goes over print work in detail, down to filing system for your computer files. While you may not need that, it makes a handy reference for everything else and is a must if you have never delved into prepress before.
Typographic Specimens: The Great Typefaces
Sure you could just print out all of the fonts you own, but this is a much easier and more complete look at the great typefaces out there.
From Amazon: First published in 1973, the Graphic Artists Guild Handbook has become the essential source for fair prices and practice. Assembled by the national organization for graphic artists, this 10th edition contains the latest information on business, pricing and ethical standards for nearly every discipline in the visual communications industry, from advertising to publishing to corporate markets.
The Business Side of Creativity: The Complete Guide for Running a Graphic Design or Communications Business by Cameron S. Foote
From Amazon: The issues most important to beginners... are covered exhaustively. Foote's discussion of fees, estimates, and billable time are right on the mark. This alone justifies the cover price.
I think this reviewer said it best as to why I include this book in the reference section: "In my opinion, this 3rd Edition "A History of GD" is the best reference book written about graphic design. This 500 plus pages book/bible is simply divided into 5 sections, 1) The Prologue to Graphic Design, 2) A Graphic Rennaissance, 3) The Industrial Revolution, 4)The Modernist Era, and 5) The Age of Information. The topics range from the invention of writing to, creation of new typogrphic styles to, the digital revolution and computer art. There are plenty of graphics and photos on every page to accompany the wonderfully written text. If you study art history or graphic design, I think this would be a great reference book to have. It will take some time to read the entire book. But this is like a text book, so, reading only part of the chapters would be quite informative as well."
HTML 4 for the World Wide Web: Visual QuickStart
Quick, easy and affordable, HTML reference.
ActionScript : The Definitive Guide
Great ActionScript reference.
Webster's New World College Dictionary,...
...because you just need one. This one's a manageable size. Aside from looking up words, try thumbing through it and use a word as a brainstorming jump off point.