On the advice of a good friend, I bought The Essential Superman Encyclopedia, written by Robert Greenberger and Marty Pasko.
I highly highly recommend it for any Superman or Supergirl fan. The encyclopedia is really an in-depth look at the entirety of Superman’s history, in all of its continuity.
Unlike something akin to Who’s Who, where only current continuity can be discussed, the Encyclopedia recognizes the worth of the older stories and older versions of the characters and locations. Entries do a great job of discussing ‘pre-Crisis’ Earth- 1 and Earth-2 histories, post-Crisis history, and then post-Infinite Crisis events.
As an example, if you look up Reactron you get discussions about his time in Daring New Adventures of Supergirl, how post-Crisis he became a Power Girl villain and battled the Suicide Squad, and then the entry reviews his current incarnation all the way up to his death in War of the Supermen. Not only are these entries incredibly detailed, they also include issue references so you can go find and read the relevant issues. For a amateur comic historian, it is a gold mine of information.
Here is the opening page about Supergirl. The Supergirl entry is a whopping 8 pages long! It includes discussions on every incarnation of the character from the Jimmy Olsen ‘magic totem’ created Supergirl to the current Kara Zor-El. No version is overlooked so even the Earth Angel Linda Danvers Supergirl gets treated with respect. The entry ends with the events of War of the Supermen so it is very current.
The entry is a very nice primer on the character and treats her with the respect she deserves.
The end of the entry also discusses all the ElseWorld versions and possible future versions of the character so it is quite comprehensive.
To get a sense of the entries, I included this opening page of Zor-El as well.
But the truth is even the most obscure piece of Superman mythos is included here. The Phantom Zone entry has a list of every criminal ever named as being in the zone and includes their crime. And characters that appeared once in the Golden Age even get a small piece.
The book is only $30 and is 500 pages of Superman goodness. It would be a perfect holiday gift for anyone who is a fan of Superman or Supergirl.