There's literally hundreds of books, I'd like to focus in just a bit - more like 2-5 books that would give me a good base of information on Genghis Khan and his reign.
I'll throw out a recommendation for Genghis Khan and the Making of the Modern World.
I myself have really only read in-depth the books related to his conquests in Asia, so if you're looking for his western campaigns I'm afraid I can't add much more.
Genghis Khan and the Making of the Modern World - Weatherford, J.
This book covers both Genghis Khan and his successors, and their influences upon different eras after the Mongol Empire. It's interesting in that this has both informative and controversial/debatable concepts, so I'd advise you to take it with a grain of salt. It's also worth noting that the area that covers Temujin's life only goes for half of the book, but the rest covers the succeeding Khans and the eventual collapse of the empire - still something I believe is worth reading. The book also tends to lean towards portraying Genghis as a calmer, more reserved character than most other books, going against the common notion that Genghis was a savage barbarian - for me, this meant that at times it felt as though the author was writing a novel rather than a history book, but that's probably more personal opinion than anything else.
Genghis Khan: Life, Death, and Resurrection - Man, J.
Unlike the previously mentioned book, this one covers a bit more of the background of Temujin's rise, including the early history of the Mongols, his unification of Mongolia, and the resulting campaigns into China and East Asia. Again, like many modern books, it attempts to portray the Genghis as both a murderous brute as well as an imperialist tactician - the author does this pretty well actually, and I think it's one of the better written sources of Genghis Khan's life.
The Cambridge History of China: Volume 6 - Alien Regimes And Border States 907 - 1368 - Franke, H. & Twitchett, D. C.
Not much to be said here. A comprehensive source more based on China, but this volume dedicates a fair bit to Genghis' campaigns into China, his tactics, and the resulting establishment of the Yuan dynasty by Kublai. More for China than for Mongolia (hence the title).