Ok, I realise that this is perhaps the most general question ever, but I want to try my hand at writing fantasy and i'd like to get at least a general understanding of how warfare worked around either medieval times or ancient times.
I don't necessarily want a book that is specific to either of those but I would also gladly read any that is, so long as the point of entry isn't extremely high. (as I am very much a beginner when it comes to this sort of thing)
Thanks in advance!
Speaking as a fellow writer, since you're doing it for that purpose, we can proceed a little differently. (For serious study into the field, I'd have to defer to the appropriate flairs in that regard.)
First, free stuff. By which we mean stuff that's on the sub. A look through the Military History section of the FAQ will be useful, especially the Ancient and Medieval Warfare sections. If you like, I can also PM you my Medieval Armies Compilation of Medieval warfare posts, some of which I really should incorporate into the FAQ.
For your purposes, we can start you off with Osprey books, as the colour plates and the focused nature make them easier to digest. Osprey can be hit-or-miss, so be cautious about the author; in this case, I recommend David Nicolle, specifically his European Medieval Tactics, Part 1 and Part 2. I'd especially recommend Part 2, as Osprey offers it in digital format as well, so less hassle. They'll give you a decent rundown of what warfare looked like in Europe from the period 450-1260 (Part 1) and 1260-1500 (Part 2), and thus you can establish your general Medieval picture from there.
Dont know about military stuff in deep, but I would recommend some sources that address the topic directly or indirectly. All of what i mention here are well known books from different epochs. Classics like Thucydides History of the Peloponnesian War and Herodotus The Histories (i had a blast reading this one in college). Julius Cesar also wrote some about hes deeds, The Gallic Wars. Also, the great arab classic One Thousand and One Nights (not much specific of military on this one but worth reading some of the short tales).
You could check about the Inca the chronics of Garcilaso de la Vega Comentarios Reales de los Incas. And the conqueror of what we know as Mexico today Hernán Cortéz letters to the emperor Carlos V Cartas de relación (i also had alot of fun reading this one).
All of this are probably easy to find on internet free. Now, if you want the historian or expert insight try to find ones that have a critical introduction.
Good luck whit your fantasy writing!