In 1954, DC halved the page count of its World's Finest Comics and forced Superman and Batman into the same book as if they were a couple of recently divorced bozos sharing a flat. They mostly kept to their own sides of the room, but a new format was inadvertently born: the team-up. This was distinct from crossovers or guest appearances because both heroes were on equal footing in a shared title with both their logos on display. Pretty soon, team ups of everyone from Green Arrow and Martian Manhunter to Supergirl and Wonder Woman to Aqualad and Robin to Richie Rich and Casper to Spider-Man and Dracula (!) began proliferating funny books.
Team-ups became so common that Michael Eury's The Team-Up Companion often reads more like a history of comics in general than one with any particular focus. The book provides a good idea of how DC and Marvel progressed from the fifties through the eighties, even as this history is presented through the lens of team-ups. I found this story most interesting when the book didn't just read like a string of synopses and the focus was sharpest, as it is in a fascinating chapter on how Carmine Infantino and Stan Lee buried the hatchet to create the first Marvel/DC team ups or a slightly incongruous but highly entertaining one on The New Scooby-Doo Mysteries, the TV cartoon in which Scooby's gang hooked up with the likes of Batman and Robin, Mama Cass, Davy Jones, Sonny and Cher, and the Harlem Globetrotters. I also loved the funny sidebars on other team-ups outside the comics world, such as Steve Austin and Jamie Sommers, Godzilla and Megalon, The Brady Kids and The Lone Ranger, He-Man and Superman, Queen and Bowie, Elvis and Nixon, Frankenstein and Dracula, and even more diabolically, Diana Ross and Lionel Richie, who joined forces in 1981 to make what I'll forever contend is the worst recording of all time.
The actual comic team-ups could get pretty zany too, as The Thing and Brother Voodoo allied themselves against Idi Amin and Superman faced himself both as Clark Kent and his own Golden-Age. That kookiness makes Team-Ups a particularly entertaining overview of Silver and Bronze Age Comics, and Eury surfs that spirit with his humorous and often personal writing.