The Phantom Zone, the place where Kryptonian criminals were consigned to served their sentences after adjudication became a rich and vibrant source of many classic tales. One of the finest characters to inhabit the Zone was Mon-El the Daxamite who had powers nearly identical to Superboy, but who had to be put into the Zone to protect him from his sensitivity to commonplace lead.
Like all gimmicks dreamed up during the Weisinger era, once it was developed it appeared with a high degree of regularity and invaded all the Superman family books. Courtesy of the writing of Edmond Hamilton, Robert Bernstein, Jerry Seigel, Leo Dorfman, and others there was a lot of wrinkles in the rich Phantom Zone concept, and lots of criminals waiting their turn to sneak out and cause trouble. The artwork by Curt Swan, George Klein, and George Papp codes these stories perfectly for their mostly early 60's era.
I've talked about this before here, but the main reason I picked this collection up was to get a good reading copy of my first Superman comic, issue number 205 (under an uber-exciting Neal Adams cover) which introduced the infamous Black Zero the villain who really blew up Krypton. It's a slam bang story by Otto Binder and the recently deceased Al Plastino, a memorable story which alas has disappeared like almost all the Weisinger innovations from the Superman mythology. But not from my own personal mythology by any means.
Here are the covers of the comics from which the stories in this collection are taken. Most are cover featured. As far as I can tell all the covers below are by Curt Swan and are inked by either Stan Kaye or George Klein, save for the last one by Neal Adams.