Didn't we have a /k/ book thread stickied not too long ago? Whatever happened to that? It was a nice archive of very interesting shit I would have never gotten around to read anyways.
Anyways: the most /k/ book I know is Karl von Clausewitz's masterwork "Vom Kriege". It's an explaination about what war actually is and goes into great detail about how wars are started, fought, and won. It is timeless and doesn't rely on focusing on the technology of the time, and instead boils every conceivable necessary part of warfare down to the very basics. Hand this book to a neanderthal, a medieval lord, the pope or the leader of the Rebellion of Mars, they will be able to wage war more efficiently and brutally than before.
It's not something you sit down in an afternoon, or even in a month. Reading Vom Kriege in one go is like drinking a bottle of wine in one go. You will taste every last drop, but after a couple hundred pages your mind will be numb and it will all be wasted on you. Sitting down and reading a chapter every month or two, thinking intently about it, absorbing every detail and thinking over details, or criticizing certain aspects will make it not only a much more delightful, but also a more enlightening experience.
Also keep in mind, that this work is not meant to be a battlefield guide. It's intended as a book for generals and politicians, who have to consider when and how to go to war. Unless you are the commander of a large military force, this book will not be of much use to you.
Wikisource has Vom Kriege (translated from German to English by Colonel J.J. Graham) in it's entirety. You can fit on any thumb-drive, or print yourself with minimal effort.
https://en.wikisource.org/wiki/On_War
(see left side-panel, section "Download/Print")
>>621947
>Mein Kampf
Fuck off /pol/. Mein Kampf is terribly written and has nothing to do with /k/.