Is Dexterity useful at all for melee combat or ranged damage (aside from letting you use more powerful bows)?
Also, any tips on melee fighting multiple opponents? My Level 11 Novice gets slaughtered by even 2 skeletons, or 4 wolves, since only one can be concentrated on at a time.

In Gothic, raising Dexterity allows you to more accurately hit targets that are further away. In Gothic II Dexterity also contributes to damage. You can read the specifics of the damage system in this post.
As for multiple opponents, when possible you should draw them away one at a time. Most creature types have a warning sound and animation they repeat two or three times before attacking; if you watch while they do this you can see how many have been alerted, and this gives you time to move in such a way that only the desired number of enemies are going to attack you. Some enemies are "linking" (such as orcs and wolves), i.e. when one spots you the whole group will be alerted, while others are "non-linking" (such as scavengers and lurkers) and can be drawn away one at a time no matter how many there are in the group; note that firing magic or a ranged weapon at a non-linking enemy will attract the whole group as if they were linking. Orcs, skeletons and humans do not give a warning before attacking.
Skeletons are the toughest foes in the game, so it is no surprise that you are having trouble with them. Before taking on skeletons you should ideally raise your chosen combat skills to the maximum level available. With "master" level in a melee skill, a heap of strength and the best weapon you can get in that chapter a few left-right-left or right-left-right combos will make short work of any enemy that isn't blocking; when you block an enemy's attack that enemy is left vulnerable for a brief moment which gives you time to start a combo. While an enemy can block a combo the master level combo is so fast that the enemy may not get the chance to block it properly.
You can only attack one enemy at a time, but with high-level abilities and a heavy-damage weapon you can take off a chunk of one enemy's health and then adjust your position if necessary. Two-handed weapons have an advantage against multiple opponents because they make an enemy jump back even if you are not targeting it (unless the enemy is to your side or behind you, in which case it will attack you without fearing your weapon), and the greater range means that in many cases you can combo an enemy at the edge of your range, damaging him and forcing him to stay at the edge of your range where he is less able to hurt you. The longer the weapon is the better; if you have to choose between an axe that does 80 damage or a sword that does 75 damage the sword is the better choice because of its length.
If you are using a bow to take down enemies you will want to maximise your bow skill and Dexterity; at the highest level you can snipe enemies from a very long distance, meaning even very tough enemies will be killed or seriously wounded before they can close to melee range. You will also want to have a good amount of Strength and melee skill to back up your archery because you will inevitably be faced with situations where enemies are too close to shoot at.
Raising your overall character level is also advantageous because you get additional hit points with each level; even if you don't spend the learning points immediately the health bonus will make a difference.

Thanks for the Info!
So far I've only used the bow to rain down death from an untouchable position, or to kite slow-movers like those firebreathing lizards. I think I'll concentrate on melee, at least until I get a good crossbow (just started chapter 2.)
Didn't realize weapon length was an issue... Good to know. Should I just drop two weapons side by side to compare? :) Currently using a mace (@36 power IIRC) I found in the Old Mine in chapter 1. Have a 60dmg two-handed weapon picked up from an orc, but waiting till I find a skill trainer.
Are skeletons really the toughest opponents? Orcs, Shadowbeasts, and even biters/snappers have caused me far more problems in Chapter 1 (yeah, i know shouldn't have messed with them.) 1 vs 1 I can take a skeleton easy (One-Handed mastery). It never even gets a hit in against the left-right-left... combo. It's when a second skeleton joins the fray that I'm dead, unless I find a narrow door/corridor where they can't easily surround me. Which btw is the way I cleared out that tower/cave not far from the Sect camp, and the cave between New Camp and orc territory (except for the Guardian zombie which I can't even scratch at this point.)

You can visually see the length when the weapon is equipped, although it's a bit difficult to compare one- and two-handers this way. You can see the length of any weapon on this handy list (the "range" column).
Skeletons are most dangerous when in pairs because they are very strong and use the fast fighting style of humans (rather than the slower, more predictable style of orcs). If you are fighting just one you can of course block and counter-attack just like against a living person. You will see pairs of skeletons more often as you proceed in the game, however, so they become an increasing threat.
As for the tower, that's part of an optional quest. If you read the book (found on the skeleton mage's body) it contains a clue for the location of another book. The quest is an interesting treasure hunt and is worth a decent amount of experience.
The guardian is killed as part of another quest which will be much easier since you have already killed his skeletons (although activating the quest might spawn a few more as is done for some quests).

Hmm, just started Chapter 3 so now I can train dual-handed... but 30 skill points for Trained??? I'm guessing Mastery will be 50-60, but even if its only another 30, I wonder if its worth it at this point. If I just put all those points into Strength, I'd be doing a whole lot more damage with one handed than with two handed, right? Is range really all THAT important?
Maybe I'll just wait a few more levels (level 15 currently with only 12 spare skill points) before going for dual-handed....

The overall most damaging weapons are two-handers (the most damaging one-hander in the game deals 85 damage and can only be acquired very late in the game, while comparable two-handers can be acquired as early as Chapter 4--or was it Chapter 3?). Additionally, near the end of the game you get a special two-handed weapon needed for part of a quest, so having at least Trained in two-handed combat is a must.
I have never had too much trouble getting enough experience to maximise one- and two-handed combat as well as high Strength and a reasonable amount of bow skill and Dexterity, but then again I crawl over every inch of the colony, complete every optional quest, and so forth. Late-game quests and enemies are worth far more experience than those earlier on, so you can expect to go up many levels in chapters 3 and 4 alone.

Regarding damage, I guess the question is if 1 point of Strength gives the same bonus as 1 point of Weapon power. In other words, if I have a 40 power 1-handed sword and a 60 power 2-handed sword, would putting 30 pts into Str end up giving me more damage with the one-handed than if I used those 30 pts on training two handed and switched to the 2hander sword?

At the point you are at, spending those points on strength is far more useful than spending it on two-handed training because the strength bonus is added to each and every hit; the weapon's base damage is doubled for critical hits, but it's better to deal a good amount of damage all of the time rather than mediocre damage most of the time and heavy damage only when you score a critical. Weapon training increases the overall speed of all attacks, which as such increases the amount of damage you can deal within a given period of time; the critical hit bonus that comes from training is useful but not crucial.
It is only in chapters 4 and 5 that you can get the ultra-powerful two-handers that measurably surpass any one-hander, and by that time you may have maxed your strength and have some learning points left to play around with (unless you are a mage, in which case you need a heap of learning points to spend on mana and magic circles).

Heh yeah, I'm in Chapter 4 now and have reached the Strength cap. I also have 3rd Circle magic, but it's strictly backup, I'm not planning to be a mage.
Thanks for all the advice and info!