In addition to the book list, I'd recommend the following:

Leo De Hartog's Genghis Khan: Conqueror of the World

Paul Ratchnevsky's Genghis Khan: His Life and Legacy

Both of these books are readable and well organized. You'll find most of the same information in both of them, but De Hartog's book is much shorter, though no less detailed than Ratchnevsky's.

If you are wanting an entertaining read about Genghis Khan and the Mongols, from the book list Genghis Khan and the Making of the Modern World by Jack Weatherford is the book for you. This recommendation is to be taken with a grain of salt. Weatherford isn't exactly a historian and his book is really considered popular history. I'll readily admit I've read it and I've used it for a source on this sub before, but you have to remember it's really entertainment. The other two books I listed above have more accurate information than Weatherford. I'd say read one of those if you want accuracy over entertainment.

There is a book list, linked on the sidebar. Their Mongolia section has a number of books on Genghis Khan:

http://www.reddit.com/r/AskHistorians/wiki/books/eastasia#wiki_mongolia