I'm a huge fan of Amazon.com. HUGE. I can't shop there because I'm Canadian, and anyway I love my local booksellers, but the customer reviews are invaluable to me.
Not so much for fiction, obviously. The business of people spiking in for 5-star reviews to support a friend or 1-star reviews to hate on a not-friend makes it difficult to judge a novel. But for nonfiction: amazing. I love that there are readers, experts in their fields, who will weigh in and compare a book to others on the market and give advice on whether it's a good buy and for whom. Those reviews are plenty long enough to get a sense of the writer's bias, an important filter in whether or not to take the review seriously.
I just wish Amazon was set up to let other shoppers e-mail those reviewers because I would love to be friends with some of them. Which, now that I think of it, is probably why Amazon is not set up to let shoppers e-mail their reviewers.