Batman: The Black Casebook is a collection of 1950s bizarre Batman stories that Grant Morrison used in his Batman RIP storyline. Basically he was looking at the same issue of “What if all this crazy crap actually happened to Batman in one lifetime? Even the batshit insane stuff from the 1950s?” that Neil Gaiman looked at in Whatever Happened to the Caped Crusader? The Black Casebook is a testament to how batshit some of those 1950s stories were.
We’ve got Bat-Mite, Batman pretending to be Indian Chief “Man of the Bats” (seriously terrible), a pile of ridiculous international heroes inspired by Batman, an alternate universe where our Batman has Superman powers, and more. The stories are ridiculous, but it is interesting that Grant Morrison used bits of them to tell a contemporary tale. Interesting doesn’t mean entertaining though. I’d skip this unless you’re a real hardcore Batman aficionado.