I just finished "Good Times Bad Times", and just started "Shadows Taller Than Our Soul." GTBT is almost strictly a pictorial history. That said, while it doesn't offer anything that Zep fans don't already know, some of the rare/previously unpublished photos are worth the price of admission.
"Shadows..." is more in depth, but again, nothing really "new" for Zep fans. What sets "Shadows..." apart is the really cool interactive stuff included in the book: reproductions of ticket stubs, tour books, press releases, stickers, etc., as well as rare photos and a CD interview with James Patrick Page.
For me, still the best LZ book by far is "Led Zeppelin 1968-1980" by Keith Shadwick. Rather than dealing with the tawdry offstage excesses documented in "Hammer of the Gods", the Shadwick book (and "Shadows...", for that matter) is much more focused on LZ's music.
If I had to choose bewteen GTBT and "Shadows...", I would definately go for the latter.