The 2 books put out by Hobby Japan The Pictorial book of Godzilla I and The Pictorial book of Godzilla II are excellent. I features eye-popping shots of every G suit from the original up to Space Godzilla, and II features all other Kaiju from the Toho films. They are quite spectacular and thorough (made with the modeler in mind).
I also like the books Godzilla: The First and Godzilla: The Second. The First features Gojira and Raids Again, and The Second King Kong vs. G and G vs. Mothra. Both have beautiful (often full-page), b/w pictures from each film, with a section of the promotional pasteups, and then many rare, behind the scenes shots. It's also nice that what little text these two have is in both Japanese and English.
There is another I have that's a "TV Kun" book that came out at the time of Godzilla: Millenium that's great as well. I've got some info on it in a back issue of "G-Fan"; give me a day or so to find it.
Also, if you like the mecha from the Toho kaiju universe (and how could you not?!), the older book Super Weaponry in the Toho SFX Films is great (one of my faves!). The more recent Toho SFX Mechanic Chronicle is also good, and still available from hobbylinkjapan.com (I believe).
Which brings us to the bad news: most of these are OOP, to my knowledge. hobbylinkjapan.com used to sell Godzilla: The First and The Second (it's where I got mine), but the last time I was there they were back ordered. Perhaps they'll get them in again (?). Drop them an e-mail and ask. The 2 Hobby Japan books are 8 or 9 years old and not available anymore. I found one at a local comic book store (where I also found my copy of Super Weaponry in the Toho SFX Fims) and the other on eBay.
These are all available at many conventions, no doubt, and ALL worth tracking down. You may no be able to read the Japanese (I can't, at least), but you'll have an excellent visual reference library of the Toho Godzilla and SF flicks. Happy hunting!