You're probably thinking of On War by Carl von Clausewitz.

The other canonical general book on warfare is Sun Tzu's Art of War.

Are there any periods you specifically have an interest in, because there are about as many books written on war philosophy as are written on wars themselves.

But if I were to recommend some books on current mainstream military philosophy, the list would be something like:

Carl Von Clausewitz's On War is the book. You can get it through almost any technology you want, its got a half dozen solid translations in print, you can get them on kindle, its up for free on internet. This is the book, and I cannot stress that enough. Since 1871 this book has captivated military thinkers across the globe. If you are interested in theory, read it. Youll be glad you did.

Achtung-Panzer by Heinz Guderian is a classic on the employment of tanks. Guderian wrote immediately before World War Two, and he described both the history of the tank in World War One, and his vision of the tank's future. Specifically, he thought made several recommendations which were adopted by the Germans right before the war. Most importantly, Guderian created a combined-arms Panzer division, which is the general template for tank formations even up to this day. As such its a noteworthy development in tank philosophy.

Gulio Douhet's The Command of the Air is a strange little book. Douhet was one of the early major proponents of air power between the wars, and he is generally regarded as the "father" of strategic bombing. Importantly, Douhet discussed how bombers would win the war of the future, and that massive bombing raids would level enemy cities, devastate their industry, and quickly drive their citizens to revolution. Douhet's analysis was a little too pie-in-the-sky for World War Two, it turned out that only very specific targets could damage enemy industry. Further, most populations actually became determined to continue war in the face of heavy bombings. But Douhet gained new relevance with in the Atomic age, when bombers had the capacity to devastate cities with little investment. So if you read his work, understand that today weve replaced his bombers with ICBM warheads, and with that perspective, a lot of what he said rings true.

And as a secondary source supplement, The Evolution of Nuclear Strategy by Lawrence Freedman discusses the most modern of military strategies, the employment of Nuclear weapons. Importantly, the book discusses how those strategies have changed over time, which is nice to see.

But there are a lot more things I could just rattle off, like this book Ive never read. But I dont know what you want to read, and thats really critical to getting through some of this stuff.

Patterns of Conflict by Colonel John Boyd was written after WW2 and dealt with Blitzkrieg tactics. The strategies and tactics detailed have been used by modern forces and is often called the modern "Art of War".