Penguin Publishing has discounted a bunch of their books down to
$3.99-$4.99 at the US Kindle site (haven't had a chance to check the other bookstores).
This is a bit unusual since I've hardly ever seen any discounts from them. There's more than 100 discounted books to wade through, many of them nonfiction. Here's an
Amazon search link for those published by Penguin and search links for those published by Penguin imprints
Plume,
Perigee,
Berkley,
Portfolio,
NAL,
Tarcher,
DUTTON CHILDREN'S,
Avery,
Gotham Books and
Riverhead to browse through the bargain books.