The simplest way to add books to a Mendeley database is to use WorldCat and Import to Mendeley, an add-on that works best with Internet Explorer. I haven't been successful using it with Chrome. But with IE, the process is terrifically easy. Search for the book (author, title, or ISBN, etc.) and then click the "Import to Mendeley" bookmark. For whatever reason, the programmers at Mendeley have decided not to incorporate ISBN lookup into the program itself, as is the case with Refworks (via RefMobile). What I'm learning is that, as a dedicated user, I will have to develop my own workflow for making best use of valuable software.
Another feature I'm playing with is SEARCH. Naturally, Mendeley allows users to search their own libraries using various levels of filtering. But as a social networking application, Mendeley also searches all publicly available Mendeley libraries. Adding a citation to your own library is a click of a link away, and, when OpenAccess, you get the paper, too.