apart from Amazon there are some reasonable book platforms that should be well considered to integrate them in the next versions:
Bookrix.com / .de - an indie- / self-publishing ebooks platform with tons of ebooks you won't find on Amazon (unless the users there share it on Amazon as well)
www.bookrix.com/
Booklooker.de - probably the biggest second hand book selling platform for the german speaking market - thousands of bookshops, libraries and second hand bookshops - as well as many publishing houses - use this platform to sell their books - for a very reasonable price (I bought already about 100 books there
www.booklooker.de/