When I buy a strategy guide, I have certain expectations. First I look for the lists. There should be lists of items in the game, lists of unlocks and their conditions, and lists of various statistics. Then I seek out the tips and tricks - advice on how to play the game better, legitimate play strategies for beating enemies or accumulating in-game goodies. Next there should be cheats, including such things as in-game backdoor codes, cheat device codes, and game glitch exploits. Finally the guide should include one or more walkthroughs, step-by-step instructions on how to get from the beginning of the game to the end.
By these standards, Prima's guide for Gran Turismo 4 falls short. It's got plenty of lists and statistics, but its gameplay advice provides little more than does the in-game text, and nary a cheat nor a walkthrough is to be found. In the case of cheats it can be argued that there really aren't any for Gran Turismo 4, and anyway one wouldn't expect to find them in an official game guide. Gameplay advice and walkthroughs are a different matter.
I suppose that it was a wise decision for Prima not to sell the book as a strategy guide, for there is precious little of that in it. As I mentioned, the gameplay advice is pretty much what can be found in the game itself. There is no walkthrough of any kind. Sadly, I was able to find more and better advice for free on the Internet than in this $17 book.
I took that free knowledge and did a little research of my own. I replayed GT4 from the beginning and took notes. You can find the results here in my blog article, The Lazy Gamer's Guide to Gran Turismo 4.
A final note: If anyone is wondering whether the official guide for Gran Turismo 3: A-Spec is any better, well, it isn't. In fact, the two guides are so similar that they look like they were authored by the same people, despite being from different publishers.