If you are looking specifically for GTR info, you'll find there are only a few chapters in the book - roughly 40-50 pages on the GTRs. The coverage is fairly slim unfortunately but it's about the best you will find short of the Michael Cotton book specifically on the GTRs. A copy of that book recently sold for about $300 on Ebay, so if you think "Driving Ambition" is expensive...
Anyway - most of "Driving Ambition" is about the development and build up of the McLaren F1. You really get to see a huge amount of that process - the photos and level of detail are quite incredible. It really is an excellent book - one of the greatest single-model-specific automotive books ever published.
If you are only looking for copies in places like Ebay, then you should stop. The book was recently reprinted by Virgin Books and is available at somewhat normal (given the value of the dollar) prices these days. I think Amazon is offering new copies for about $60. You can also buy slightly used copies through them for a bit less, so you might want to go that route.
>8^)
ER