Recent publication of the two foremost experts are also noteworthy, though addressing more broader issues than the military conquest in a narrower sense and focusing also on Chinggis's successors than Chinggis Khan himself.

• Jackson, Peter. The Mongols and the Islamic World from Conquest to Conversion. New Haven: Yale UP, 2017.
• May, Timothy. The Mongol Empire. Edinburgh: Edinburgh UP, 2018. The Edinburgh History of the Islamic Empires.

 

As already noted by /u/wotan_weevil, the diversity of the relevant primary written sources and their languages are the first problem for those who are interested in the history of the Mongol Empire.

 

Rossabi, Morris (ed.). The Mongols and Global History. New York: Norton, 2011: is a handy collection of the translated exerpts of such sources, probably intended for the undergraduate course students in Anglophone countries, but this collection is a tip of the iceberg of the bulk of the sources. In addition to the famous Secret History of the Mongols, especially some Persian sources, such as Juvaini (d. 1283)'s the History of the World Conqueror (linked only to the old English translation) and Rashiduddin Fazlullah (d. 1318)'s Jami'u't-Tawarikh (Compendium of Chronicles), have recently attracted more attractions from researchers (and English translations are available).

I am especially interested in their tactics and strategies employed in conquering and their troop deployment(not necessarily to focus on these facts, but I'd love details on them).

For this,

• Carl Fredrik Sverdrup, The Mongol Conquests: The Military Operations of Genghis Khan and Sübe'etei, Helion, 2017.

IMO, by far the best on this topic. Sverdrup makes use of a broad range of sources, both Persian and Chinese (and, as far as possible, Mongolian), and discusses the limitations of these sources and the resulting uncertainty.