I like how why's (poignant) guide to Ruby kicked off the trend for quirky programming language books, and now we're back to writing quirky programming language books for Ruby. (Though the poignant guide may have been a little too quirky.)
Writing a book and allowing your personality to come through is more interesting than making it quirky because it's fashionable to be quirky.
Anyone old enough to grow up in the era of early PCs remembers quirky programming books. I learned Pascal from the Sherlock Holmes-themed Pascal book back in the day. Though, admittedly, Mr. Bunny and the cartoon foxes are an order of magnitude quirkier.