First, consult the book list. These books are very good starting points for someone interested in the Mongols and Genghis Khan.

I would recommend the following if you are interested in any of these subjects:

Genghis Khan and His Descendants

Francis Wood Cleaves' The Secret History of the Mongols

This is among the first translations of the Chinese version of The Secret History of the Mongols. It is a bit hard to read, but if you want some hands on document reading regarding the Mongols, this is a good place to start. If you have access to JSTOR or Academic Search Complete, you can probably access a free PDF copy of this work. I highly recommend it.

Bertold Spuler's History of the Mongols

Spuler was one of the leading historians of Genghis Khan and the Mongol Empire. His work influence numerous other historians, including Rossabi and De Hartog. This work is very hard to get through, but a very good book regarding the Mongol Empire.

Morris Rossabi's Khubilai Khan: His Life and Times

Rossabi's work isn't flawless, but if you want to know about the REAL Khubilai Khan, not what has been popularized on the Netflix Series Marco Polo, this is the book for you. It is easy to read and really takes a look at Khubilai Khan through primary sources. It also talks about his patronage of the arts and sciences.

Paul Ratchnevsky's Genghis Khan: His Life and Legacy

I would argue that this is one of the best works on Genghis Khan. It is easy to read, though it reads like Great Man history. Ratchnevsky details some of the bad things about the Mongols, but he doesn't really go into much detail.

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

This is a concise and very well done biography of Genghis Khan. I would recommend this over Jack Weatherford's Genghis Khan and the Making of the Modern World. Weatherford's work is really good, but historians are not convinced that it is a piece of pure scholarly work. This biography is very, very good. It is short, but has a lot of detail in the sections which require it. If you are looking for a narrative about the major events in Genghis' life, this is the book you should read.

Mongol Military Tactics

Timothy May's Mongol Art of War

This book gives an accurate overview of Mongol military campaigns and tactics. The only problem with this work is it isn't presented in chronological order. May skips between campaigns. He does provide an interesting section on what the Mongols took from each military campaign. It is useful for the reader to understand how the Mongols thought about war.

Denis Sinor, "The Inner Asian Warriors," Journal of the American Oriental Society, 1981

This article was written by one of the most prominent Mongol Historians, Denis Sinor from the University of Indiana. This is a great companion article to Timothy May's book. It is not very long, but it has some of the best information about Mongol tactics, strategy, and methods of war.

Richard A. Gabriel's Genghis Khan's Greatest General: Subotai the Valiant

This book is a little bit suspect because it was written based only on secondary sources. Honestly, it reads like a movie script. That is good if you want to be entertained, but not so much if you are wanting to seriously learn about a person. This is a perfect example of Great Man history, as it is a biography. I would still recommend this book to the casual learner of Mongol military history.

Hope that helps!