In an article on Tech Republic, an author listed what he (or she) considered to be required reading for a geek. While the list is a decent start, there are many books that I felt were left off of the list that are excellent reads for geeks.
Lord of The Rings trilogy: Any book which defines a new language is bound to be popular with geeks, but the fact that this book is the Grand daddy of all modern Fantasty books, and you have a book that is almost be required reading for every geek.
Myst: This book is the prequel to the story told in the popular mid -90s video game. I've always thought of the clues and game play of Myst to be particularly geeky and the prequel in book format does not disappoint.
Flat Land: The story of an imagining 2 dimensional World and what happens when it encounters beings from a 3 dimensional world. The topic and the Science involved are certainly in the realm of the Geeky.
Foundation Series by Issac Asimov: This Book is based on the premise that by studying data enough, one could predict future events. Place a story regarding this in space, and you have the Holy Grail for a geek sci-fi book. This book has several sequels, but I feel the first book was by far the best.
This of course is not an exhaustive list, but does contain several good books that should be included, or at least considered for a "geek" reading list.