I get the feeling from posts I have seen throughout this site that there are some strong feelings about not looking through your opponents army book to understand how to fight it. Why is that? Seems to me as you play more games, you learn it anyways. To me this is similar to not looking at what spells your opponent has and the rules that govern them. What is your take on studying other army rules?
I'm all for it, as long as you're not wasting valuable game time doing an in-depth analysis of their units why they're waiting for you to move your models. I play a lot of 40k against Grey Knights and Blood Angels and it's getting to the point where I know the Grey Knight player's army just as well as I know mine, and he reads up my army's rules as well. Often we'll swap lists and check each other's stuff before submitting them for tournaments. That all said I'm not a big fan of list tailoring. You can easily get into an arms race with a regular opponent and wind up falling into the trap of having an army that beats one person's army but is terrible against everything else. It's a lesson me and my friend learned the hard way. Mostly me...
I don't have so many army books, so i cannot "learn" them. But to know the "tactica index" of other armies, is always a good thing, and here we have nice links to the forums of other armies. During play, if it's the case, I take a quick look at my opponents' book only if an ability sounds strange to me (aka, it's played differently from what I know or from what I've seen before).
I tend to read other armybooks mainly as a way to learn the the statline of other units and what their rules are. However, I find the only way I really get to know about another army is either by playing it myself or having to face it repeatedly. Sometimes, the only way to know how to deal with certain tactics (beastmen ambushers, dwarf gunlines, etc) is to face them and often lose. I'm certain some players can see ways around some tactics even before they face it, but I'm just speaking out of my personal gaming experience.
I downloaded all the rules for all the armies in aos. At the very least I can help my opponent build better armies, and knowhow to counter them! Haha! But art of war. Know thy enemy.
I've got to agree with @Lord Grok Of Xillaqua - I don't play as much these days, but when I did, I used to read through army books to get at least an idea of what I might be up against. If anything, I used to ignore other army-specific forums - tactics change with the local meta and player skill, but unit stats are unit stats. I used to think of it like knowing the enemy's typical orbat and equipment specs, but not knowing the tactics a specific commander preferred (until I'd faced them a few times, at least!), if that makes sense? I personally don't see a problem with at least knowing their basic statlines and equipment options, anyway.
Have issues reading PDF's due to my eyes being strange at times, not being able to buy all books either due to them just collecting dust plus it's a lot of money to fork out. I always play against my boyfriend, with his Chaos army. It's his sorcerer that does all the work that wrecks my army, Dwarves; his shoots my army to pieces or that one dwarf character.... With 40k, so far shooty Astra Militarium and now his Orks are close-range hitters. I seem to know just by staring at the models and layouts. Usually in the Workshop, army strongpoints and weakpoints get bounced around in conversations, I take careful note for when I decided to come out my shell a bit more and battle others more. Yet it's mostly Tau, Chaos, pretty much those types of armies.