Books that I have reviewed here, on the forums, and others that I have written, informally, are usually from the collection of books that I have bought over the years. In instances where I have been asked to review a certain book, it is usually sent to me to review. For budding writers, like myself that usually don't get paid, oftentimes we get to keep the book we review as a "form" of payment. Since it is not the publishers handing me the book directly I feel no obligation towards them to give a favorable review, though I rarely find a book without any redeeming qualities.
As for guidelines, it is really difficult to develop hard and fast rules to follow as cookbooks come in an endless variety of catagories and styles. Basically I try to figure out what the author is trying to do, the audience he is trying to attract, and the knowledge he conveys. I then judge a book by what the author is trying to achieve. Of course I want the information and facts to be correct and I want recipes that are concise and accurate. Beyond that, it is hard to pin down, as a cookbook written for professionals will be very different than those written for the home cook just as "The Joy of Cooking" is a very different book than Thomas Keller's "French Laundry". One is a very utilitarian cookbook. The other is as much as story about Keller's journey as a cook as it is a cookbook, IMO.