Why yes! I think I can help with this! :P
I'm not sure if you've played Oblivion, but I think that's what you're going for. When in town it has its own set of tracks. Same for in the field, dungeons, and battles.
So... That's exactly what I do. I have some folders on my computer divided up by settlements (divided further by city, town, village), field (normal, specific weather types, specific land types), dungeon, battle (normal, mini-boss, boss, epic boss), and mood (spooky, rising tension,...). They're all ready for me if I need them. But I take this a bit further. If you plan out dungeons beforehand like I do, you can just make a temporary folder with your pre-picked set of play lists.
Say we are traveling through the mountains to meet their NPC friend at Temple of Generic Doom. I have a "mountain travel" play list already going when they arrive. When I am about to tell them that their NPC buddy is sprawled at the entrance in a pool of his own blood, I already have the next song ready from my mood folder. All I have to do is double-click. When they enter the dungeon, I have a "generic dungeon temple" mix ready to go because it's all in one place.
I'd recommend that you do not use music with lyrics in any language that your players know. It can become very distracting.
I find that I can get the best effect from songs without lyrics. I make exceptions to this when a bard is around.
Silence can be much more powerful than sound. Remember to turn off the music every once and a while.
Find some specific tunes that you do not use in any of your generic soundtracks. Give special characters and places a theme song (make sure that your players haven't heard it before, or it loses most of the effect. Trust me. XD)
After I have written all of that, I feel that I should mention another option as well. It's possible that you do not actually want music. In fact, I prefer this method for most situations. You might consider just ambient sounds like crickets, the ocean, water dripping, a fire burning, wind... You get the picture. If you have a sound editing program like I do, you can make your own mixes out of these basic sounds. I once made a very nice piece I called "Campfire" consisting of crickets, fire, some rustling leaves, a couple of quiet conversations, and an instrument (for my NPC bard following the party at the time).
Anyway, there are plenty of sources for good music. In fact, video games and movies are going to be your bread and butter. I'm a bit of a gamer, I know quite a few video games. And their soundtracks. I'll try to leave some examples for ya if you want to look them up.
Definitely try Chrono Cross (Tower of Stars, Dragon God, many MANY more), Dragon Age (Tavern Brawl is awesome!), Morrowind(any), and Oblivion (any). I use them a lot.
Baten Kaitos has some pretty sweet boss music, but sometimes I think it's too modern to work for most campaigns. Same with Tales of Symphonia or Vesperia. It all depends on your style.
Fire Emblem and Final Fantasy are both chuck-full of awesome music too. Unfortunately very popular. As I said earlier, if your players have already played these series, do not use.
Just remember to get soundtracks for things your players haven't played/heard before. It totally ruins the mood if they know where it's from. I went out of my way to ask them what games they've played and vividly remember before I started buying the soundtracks or finding arrangements. Same deal with movies.
Anyway, I'll shut up now. XD