Over a period of 50 years, legendary Dutch designer Ootje Oxenaar drew replacement book cover spines for the books in his library. A selection of his spine replacements are collected in a book called Ootje Oxenaar Spines.
Although renowned for his designs for Dutch banknotes and postage stamps, Oxenaar was a prolific designer of book spines. This wasn't done for commercial publishers, but for books in his own library. When he didn't care for what he saw poking out from a shelf (or when he needed to procrastinate) he would make his own spine for a book. The result is a fantastic and fantastical mosaic made of tall-and-skinny strips, hand-lettered and drawn with great skill and great whimsy.
Check out Steven Heller's post at Print for more examples. (via i love typography)