Features include searching for books by the problem you are trying to solve, one-stop cross-searching of all the hacker sites so you can review what other hackers say about the book, and free startup tools like a countdown clock for your startup's runway.
About the time I did my site there were a bunch more book sites created. If I had the list I'd be happy to post those as well. Seems like books -- both free and paid -- are always in demand with this community. I think the key feature here in all of these sites is "community curated." I find reading what other hackers say an irreplaceable aspect of selecting which books to read. (My list was generated by spending a couple of days on Google searching HN and reddit finding the best-of-the-best books for hackers)