From the site:
BookBub is a free daily email that notifies you about deep discounts on acclaimed ebooks. You choose the types you’d like to get notified about — with categories ranging from mysteries to cookbooks — and we send great deals in those genres to your inbox.
BookBub doesn’t actually sell books. We simply alert you by email to fantastic limited-time offers that become available on retailers like Amazon’s Kindle store, Barnes & Noble’s Nook store, Apple’s iBookstore, and others. Book publishers offer deals at these sites for promotional purposes, and our staff works with them to determine the best ones to feature to our members.
BookBub features ebooks ranging from top-tier publishers to critically acclaimed independent authors. Our team of experts makes sure that we’re only featuring great deals on quality books that you’ll love.
Founded in early 2012 by a team of people who have worked in book publishing, book retailing, and online media, BookBub has quickly become the leading daily deal email for digital books, growing to more than one million subscribers.