The Essential Batman Encyclopedia by Robert Greenberger - A handy and good looking reference for Batman fanatics that delves pretty deep into the characters populating the Bat-books through the decades. While I've read DC comics off and on since the 60s, I'm just a casual Batman reader, but much of the content here was just over my head.
This volume is written from the contemporary DC continuity perspective, i.e. everything printed in DC comics in the Golden Age of the 40's actually happened on Earth-2; not on our Earth. I found that to be rather jarring as Earth-2 was a 1960s invention. So much of the content here is twisty revisionist. I'd prefer reading historical entries in historical context.
For example, the entry under "Supergirl" starts off talking about Kara Zor-el (Power Girl) of Earth-2 and states (mistakenly) she was introduced in All-Star Comics #57 (1951). That would be before Supergirl was introduced. As far as I know, Power Girl was introduced in All Star Comics #58 in 1976, a couple of decades after Supergirl debuted. I looked at a few other example entries and found other similar contextual (and perhaps typographical) errors, and other entries are fine if you can get past the revisionist writing.
Maybe it's just me, but it's weird to read that the Earth-2 Catwoman debuted in 1940. There was no Earth-2 in 1940! If I can get beyond that weirdness I guess It's OK.