Since I am the kind of person who can happily wander a bookstore's shelves at random for hours until someone comes and hauls me out of there, I have fallen in love with Shepherd, a new (relatively) website for book recommendations. An electronic gold mine for both readers and writers doing research, Shepherd's bookshelves are browsable by Wikipedia topic, by favorite book or author, or practically anything else you want to specify. It is perilously easy to spend much more time than you meant to wandering its virtual stacks.
Shepherd offers lists of books on practically anything, recommended by over 8,000 authors who know those subjects. Naturally that includes me because this is the kind of thing I can never resist. Each of us has collected five books to suggest on anything from the best novels about WWII to the best books on the Hittite Empire. I compiled one list on books about life in the Roman Empire and another about books on the Sixties and the Vietnam War era. Each writer gets to plug one book of our own and tell you why we chose the others on our list.
Handily, you can buy books through the site, and Shepherd gets an affiliate commission to keep it running and to develop new features. You can check out my page on the Sixties here and my page on the Roman Empire here.