My players are extremely strategic, and they're consistently (and too easily) defeating groups of enemies that I intended to present a real challenge. Probably for the same reason that I suck at chess, I suck at designing effective strategies and tactics for the enemies. Are there any books or other resources that I can read to help me make my monsters more effective in combat?
I would suggest reading up on Sun Tzu, Alexander The Great, Hannibal, Scipio Africanus, Julius Caesar, Genghis Khan, Tamerlane, Napoleon, Wellington, Clausewitz, Guderian, Patton, and Petraeus... Just to name a few. And if you only have time for one, read The Art Of War by Sun Tzu.
How to apply this to RPG situation? This is not tricky. As a GM you control nearly everything: the terrain, the weather, the intelligence both sides have, and the moral/skill of the opposition. Use those to your advantage: Sun Tzu tells you what to look at and how things affect what. Use the terrain to block the characters from moving. Use the weather to stop those arrows or spells. Use the intelligence to come up with specific weapons/counters for the players. Never underestimate the power of hit-and-run -- especially if poison in involved: Now, all your characters are ill. Shame...
Side note: It is easy for the GM to swap the character and kill them all in one round. This is (generally) no fun for the players.
Edit: Why did I not include Musashi's Book of Five Rings and Machiavelli's art of war. The first one is because it is an individual fighting manual and not a treaty on tactics or strategy. Musashi's legends state that he was a great swordsman but he never commanded armies. Machiavelli, those political work is a must read, I did not include because while as a politician I rate him very highly, military wise, I do not. That is probably prejudice on my part.
First of all, remember that the purpose of the game is for players (and, secondarily, the GM) to have fun. As such, ask yourself if your battles need to be more difficult. Do you feel the ease in which the players beat all your monsters makes them bored, or are they excited that their tactics are so successful?
Remember that the game is not you versus the players. You're all playing together, and the purpose of the game is to have fun. You don't have to beat them.
Okay, next thought. I agree that, in some ways, conventional tactics can be useful to GMs. However, those tactics were devised for situations where the participants were in the thrall of nature. As a GM, you define it. Let me explain what it means. You have essentially many ways to provide (combat) challenges for your players, some of which are:
Note that the mere existence of some of those things (without any long-lasting effort on your part) can have a profound effect on battle, and possibly, on your players' lust for life.
Sigh Okay, there's some more, but this is what I have off the top of my head. The point is, don't restrict yourself to mundane tactics. Use your omnipotence to sculpt scenarios in which you can crush your players, body and mind. Shatter their psyche, and instill in them deep-seated fears of multi-sided dice.
Edit: Hmm, this looks like an interesting project. I think I'll make a more detailed and organized list of ways in which you can make players' lives a living hell. What was that? 'The purpose of the game is to have fun', you say? Why, whoever could have told you such nonsense?
The first thing that I would need to know to properly answer the question is what game system you are using is. Effective strategy can vary wildly depending on the mechanics.
Greg has a very good group of fundamentals to start with, and some of the key to that is the party itself. Most of the characters I encounter try to stack into being able to succeed at a straight fight, because 9/10 fights are "stand and deliver" setups beyond maybe a sucker punch. Don't be afraid to let the enemy be conservative and keep a few guys back. In an L5R 3e game I ran, I had a group of soldiers that had an ability based on their initiative come up against the party. Half of them sat back and used a rule (for everyone) stating that their initiative could be increased if they weren't hit that round and by the time it was their turn to fight, they easily doubled the party's initiative and had the potential to slaughter them handily despite being half their strength.
Terrain is also quite critical to a combat. Once again with the "stand and deliver" they are often in a broad room/vast plain where space isn't an issue. Feel free to catch them in a valley or a corridor and limit how many of them can fight at a time. In a D&D game I was a player in, we were in a 5' wide corridor and wandered into a nest of spiders (somewhat) giant spiders. They were able to crawl on the walls and ceiling but could ultimately swarm each member of the party at their leisure and we couldn't do much without risking damage to our own people.
Sometimes codes of honor make all the difference. In 7th Sea, an official duel with a member of the Swordsman Guild is a one on one fair fight. If the battle is against the player, nobody else can (legally) step in and do anything about it. This battle will resolve a lot differently than being surrounded by a couple groups of thugs.
Lastly? Stealth. Once again, it's usually an issue that is overlooked very frequently. Have a guy who has highly enchanted armor or is as dexterous as Jackie Chan? Hit'em in his sleep. Magic sword or state of the art gun? Steal it. Then see how much fun they have in a fight.
So with a little more information we can probably give you something to work with.
Go read Greg's answer again.
But I'd like to add that if your players are extremely strategic, give them some different scenarios to test their flexibility and give them different goals to adjust to.
How often do you face the enemy in a open featureless plain in a fight to the death? The tactics have to change a lot if the other guy is holed up in a castle. Likewise, you'd be a fool to rush out and meet the enemy if they have a lot of cavalry and you have a really big wall. And it doesn't matter how good the rogue can tumble if you're in hallways facing an endless stream of people. Mr. Fireball is more or less powerless when the enemy is a single urchin darting through a crowded marketplace. Mr. Maginot, the dwarven defender was deeply shocked when the tigers simply skirted around him and began slaughtering the casters in back.
The goal doesn't have to be slaughtering everyone, it can be to simply bypass them somehow. Since actual combat isn't the focus, the DM can place some truly horrific monsters in play. Situations like this need to be conveyed to players. They need to learn the tactic of running away.
If you want a touch of realism, most forces will simply surrender or rout once half of them are taken out. This whole fanatically suicidal fight to the death thing is highly over-rated. And to turn that about, send them on a task to go slaughter a group that has self-preservation in mind. How will they deal with scattering targets? Smoke-bombs and sneaky targets?
Your players may be tactical geniuses, but if you change their goals, you may present new challenges to them.
To learn strategy and tactics, do as real generals have done for centuries: read accounts of famous battles, study the way they've played out, and extract your own lessons from them. A visit to your local library and a chat with a research librarian is your best resource, as the sheer number of books that fall into this category is enormous.
Incidentally, this is exactly how top-tier chess players improve their game: by studying famous and not-so-famous games.