A doodle is something more. Not a signature that's bashed out thousands of times, but something chosen and executed with imagination, something that took time and thought. It may not be great art, but it's far more personal than a signature.
There's a site Doodled Books that sells doodled books by all manner of authors. Some of them are very good, some very funny, and some even both. Anyway, it takes a helluva lot longer than a signature and they pay me £5 ($7.50) for doing one, but I enjoy scribbling and each one is a book sold and more than that it's a book someone is going to keep and look after.
So, if you want one here are the kind of things on offer. Go check 'em out!