What are some good books about the hashshashins (assassins), Alamut, etc?

Bit of a longshot as it seems very difficult to find such a book or source. The most well known book about them seems to be Bernard Lewis' 'The Assassins: A Radical Sect In Islam' but it seems to have received quite a lot of criticism. I find this group highly interesting and would love to know as much as possible about them.

Thanks.