My old FM pal Margaret Fisk tells the tale of her stalled WIP and my carrot. I'm touched and grateful, not only that she remembered, but that she didn't describe my artwork in too much detail. I assure you, that was the lamest drawing of a root vegetable in recorded history.
In the not-lame department, Neil Gaiman is said to draw neat things in some of the books he signs, but I think he's a pro graphic artist on the side, isn't he? I know for a fact that Stuart MacBride does excellent vampires, as well as cute little teddy bears carrying chainsaws and flame-throwers. Another reason to go to his signings, Europeople, you might be able to badger him into doing one for you. For you Talyn winners, Holly Lisle has already done some very cool things while signing the giveaway books.
I was never much of an ink artist, and you can't paint on book paper, otherwise everyone would be getting some of my watercolor Mutant Ninja Turkey-Swallows or The Dread Orchids of Doom in my signed copies. But now that the osteo doc and Humira have worked wonders, I can hold a pen for longer than three minutes. I'm tempted to break out my box of crow quills and rapidographs and get back into doing things like this:
Obviously I wasn't thinking much about planetary gravitational fields when I cooked up that one, but it was just for fun. What's the coolest thing you've ever seen an author add art-wise to a signed book?