The previous incarnations of the DC game have had sourcebooks. Batman (from Mayfair) was self contained and had about half the book devoted to Gotham (broad strokes tho) but there may have also been a sourcebook for Batman (updates maybe?).
The WEG version also had a Bat-book (Nightwing and Catwoman fighting Bane on the cover I think) that expanded on the basics in the core DCU book. Since a Metropolis tourist's guidebook was released, I'm fairly certain a Gotham one was released as well.
There's general info and pics in those big hardbacks in the fantasy / sci fi section of any bookstore. They've done Superman, Batman, Wonder Woman, Spiderman, JLA, X-Men and the Avengers (I think). I once saw a hugemongous encyclopedia of the DC Universe at Sam's Wholesale.