Whatever Happened to the Caped Crusader is one of my favorite tellings of the Batman story, but it shouldn't be read by people unfamiliar with Batman. I've tried reading some of Gaiman's other work and found it boring, this book shows that Gaiman is a good writer and able to bring advanced narrative techniques into comic books (though he must use other author's characters and plots to be entertaining). Due to the narration style, this review may contain what some readers consider to be spoilers.
The artwork is amazing, perhaps it's because Gaiman tells the story in such a way that each panel is sufficiently different from any other panel. Additionally the artwork changes to match the style of the stories being told. The title comic involves Batman's major enemies and ends in a way that will disappoint the most hardcore fans. There are four other, shorter stories in this collection. A Black and White World shows Batman and Joker as actors, complaining about the roles they've been playing for so long. Pavane tells the story of a psychiatrist who tries to understand Poison Ivy, though it appears he ends up going crazy in doing so. Based on the artwork, When is a Door appears to be a story entirely contained within Original Sins. Original Sins tells the story of a rich couple threatened by Batman whom decide to interview Batman's enemies in attempt to earn them the public's sympathy (this is the least original story in the collection, it's been done before). When is a Door gives a partial origins story for my favorite Batman villain, the Riddler, my only complaint is that it's not long enough.