Around 10 years ago I was one of the original product testers for the 'The Beauty Bible' and 'Feel fabulous Forever' books by Stacey and Fairley. I can certainly confirm that we blind-tested the products, to the extent that when I really wanted to continue using one of the products I had a hard time trying to identify it (turned out to be Cellex-C, by the way).
Unless the authors have changed their testing and reviewing methods for more recent editions, I'd be pretty sure the reviews in their books are genuinely impartial.