A book you can finish in a day - The Realm of Possibility by David Levithan
I'm loosely fulfilling this category...I didn't read this book in one day, but I easily could have! It's pretty short and because it's written in an experimental poetic style, some of the pages don't even have that many words on them. Not to mention, the different perspectives are intriguing!
Photo Credit: I added text to the original "book sale loot" by Ginny via flickr in accordance with the Creative Commons Attribution-ShareAlike 2.0 Generic License.