There's a a vast number of books on the subject. I don't know what you mean by "more direct" but here are some books that I have read and are generally considered classics in the study of War.

Art of War and The Prince by Niccolo Machiavelli: The Prince is his better known work but both are classics and offer some good insights into the more immediate military side of war, as well as into the political side of war

The Influence of Sea Power upon History by Alfred Mahan: This book is Clausewitz or Sun Tzu for the navy. Every modern navy has, and continues, to base their strategy off of Mahan's book. I've also heard that Julian Corbett's Some Principles of Maritime Strategy is excellent, although I haven't read it myself.

Chairman Mao's The Art of War (it's a popular title) and On Guerrilla Warfare are IMO the best books (both information and writing wise) on Modern Guerrilla Warfare. The USMC's Small Wars is also an important read as it is highly influential in US military tactics today.

Diplomacy by Henry Kissinger spends very little time focusing on the military aspects of war, but it's a fantastic book for determining WHY nations do things and what wars accomplish. It's a very enjoyable read, so even if you want to stick more on the strictly military side of things I still recommend it. IMO it should be the definitive book on Diplomacy in much the same way Sun Tzu and Clausewitz are considered the definitive works on war.

I'd also like to point out that it's very difficult to ever fully "read" many of these books. It's possible to analyze these books in great detail, and the lesser known writings of these authors can be of interest. The only one I can say to have studied in any intense way is On War by Clausewitz using this website as a guide. There are raging debates about how applicable On War is to modern warfare and about what exactly Clausewitz was trying to say. If you run out of reading material, or are particularly interested in one work, you can probably find an intense debate on the work and spend a great deal of time digging through all the opposing viewpoints.

The following two recommendations are specific to the development of the use of air power. The interwar advocates of the use of bombers to win the next war (after WWI, that is) were led by the Italian theorist Giulio Douhet. His book Command of the Air is available free and in English here in pdf format.. His American counterpart was Billy Mitchell, who wrote Winged Defense. That book is available free here. These men--along with Hugh Trenchard in Britain and Walther Wever in Germany--advocated for heavy bomber fleets that could attack the enemy interior, navy, and cities to cripple the enemy war effort and compel the enemy population to demand that the war end. These books are specific to the development of strategic bombing between WWI and WWII, but they were and are very influential to how warfare has developed in the 20th century.