I loved that series as a kid... I ended up working at the library in 1998 or so and searched the basement of withdrawn books (because I hadn't seen them on the main floor in years) but alas I couldn't find them, or the purple ones.
I have been meaning to try to pick up those books when I have the chance... some of them go fairly cheap on ebay, but of course I REALLY want the Godzilla one and that one tends to cost more.
As has been said, a lot of the information in the book about Godzilla was wrong, but a longtime friend of mine and I were talking about those books and it occurred to us... back in the late 70's when that was written, how much information would Ian Thorne have even had access to? I'm assuming the book wasn't written in conjunction with Toho (although it did have some neat behind the scenes photos), and if that's the case his information would have been limited. Not to mention he couldn't just pop in a VHS of the films to get a better idea of what was going on, although I'd assume if he was really trying to write that series he had access to film reels or something, not just going off his memory.
Then again, as accurate as they were, maybe he WAS just going from memory.