If you're looking for fundamentals, Sun Tzu's
Art of War is a popularly-memed classic on strategy, and its modern equivalent
MDCP 1 Warfighting echoes a lot of the same sentiments, but with modern clarity.
But these are very general strategy books, not tactics, and the reason for that is tactics are always evolving and must be adapted differently depending on the situation, the technology, the threat model. This is probably the most important takeaway: you must always be adapting and learning and changing your tactics in response to new information and events. In addition to understanding risk (not in an academically fragile mathematical sense), this is probably one of the most important fundamentals to understand.
I think war is ultimately just too simple to bother theorizing much about, not to mention a waste of time. It's not easy, of course, but it's simple to understand. It's in our bones. It's a part of primate nature. The only things that change are the technology and the scale, which inform the tactics.
Here's something closer to a more tactical fundamental, something that hasn't changed since Roman times:
Here are modern soldiers erecting fortifications to help hold a location of interest: https://mwi.usma.edu/effective-weapon-modern-battlefield-con.... From the HESCO barriers of today to the wooden palisades of two-thousand years ago, they accomplish the same fundamental goal. Protect a location of interest, keep your supply lines safe, etc.
Infantry-wise, like meetingthrower said, you "fire and manuever". There are nearby enemy soldiers attempting to harm our supply lines or pierce through our fortification to the location of interest? Go "fire and manuever" on them. That's the fundamental. Whether you do it with spears or rifles is another matter. It's not rocket science - the desired outcome is the same. How exactly you do it changes according to the technology, the threat, geographic features, and environmental factors.