Back in college, my friend Adam and I were a bit obsessed with an odd and intriguing book called the Codex Seraphinianus. Created in the late 1970s by the Italian artist, architect and designer Luigi Serafini, the book is ostensibly some kind of encyclopedia of a strange fictional world. And now, I'm happy to say, some really awesome person has digitized the entire thing and uploaded it online. We first learned about it in Douglas Hofstadter's Metamagical Themas and, after that, eagerly sought it out in our college's library. We found it there, and it was quite a sight--an immense volume, totally impenetrable yet beautifully illustrated. And to make matters even more interesting, all the text in the book is written in an as yet undeciphered script.
Unfortunately, the digitized version doesn't really do the book justice. I think you have to see this thing in the flesh to truly appreciate it. But if you've never seen it before, it's worth checking out online--as long as you promise to get your hands on the real thing at some point in your life.