I am a member of Librarything and also on their early reviewers group. They get a batch of advanced reader's copies of books from various publishers every month and members can make their picks on the books they would be interested in reviewing. The members are then randomly selected or at least I don't know the selection process. I have snagged two books from the July batch and actually received my first book today. Advanced reader's editions of a book are published only few in number and are given away for promotional use and also to get a sense from the reviews on how a book is going to perform when it actually hits the market. I don't know how much importance it has but I am feeling pretty special. Eh, a publisher is sharing a book with me for which only 80 copies got printed and is waiting for me to send a review so a prediction can be made about its profits as well as the popularity of the author. I can take a bow, thank you very much! By the way, I joined Librarything to catalog all the books that are in my library or my memory for that matter that I have read. Its a great place to share your love of books with other readers and make friends.
Disclaimer: I am not affiliated with Librarything in any way or doing any promotions for them, though I am sure they love it when people write about them. I am just a member with whom they share their bandwidth and yes, the advanced reader's editions too.