I'm a big nerd for design books. Perusing the design section of a good book store is pretty high up on the list of things I enjoy doing. If you have even a passing interest in design, I'd recommend adding the following books to your library. Some of them are super technical and informative and others are just really pretty.
The Art of Looking Sideways By Alan Fletcher
The Art of Looking Sideways, by the legendary Alan Fletcher, is about the connections between images and language. The loosely structured book, filled with Fletcher's unique illustrations and observations, is a fascinating window into the mind of one of the 20th century's greatest creative thinkers.
The Elements of Typographic Style by Robert Bringhurst
The Elements of Typographic Style is the ultimate typography resource. Bringhurst put together the most thorough, and well written, guide to typography imaginable. This book is an absolute must read for anyone interested in the finer points of design.
Things I Have Learned In My Life So Far by Stefan Sagmeister
Things I Have Learned In My Life So Far is really a collection of 18 small books. Each beautifully designed book contains Sagmeister's musings about a different part of the design process.
Grid Systems by Josef Müller-Brockmann
Grid Systems is a great resource for understanding how grids work. Brockmann is one of the Swiss masters, and this is the definitive book on grids.
Type: A Visual History of Typefaces & Graphic Styles by Alston W. Purvis
This books is not only a really thorough history of type design, it's also plain gorgeous. Made up of two volumes, Type, traces the evolution of typography from the 1600s through the mid 1900s.