The Myth series is one of the best young adult/adult Fantasy series around. It was started to give Robert Lynn Asprin a break from the depressing The Bug Wars and The Cold Cash War books. He started work on a humorous series about a magician's apprentice who fell in with a demon. And what do you know, it sold!
Skeeve was a magician's apprentice working to be a thief, when all of a sudden his master was killed by off-world assassins while conjuring up a demon. It went downhill form there. And then uphill. Then downhill again. Then uphill... You get the idea.
Aside from the writing style, which is surprisingly light and easy, the best thing about these books are the worlds they are set in. It all starts in Skeeve's home dimension of Klah, but he quickly learns that there are other dimensions, through which one can travel. His new master, for example, comes from Perv, which is populated by humanoid lizards (yes, Skeeve finds this disturbing). The coolest of these dimensions is Deva. In the distant past, the entire dimension underwent a massive economic collapse; the inhabitants (deveels; they have red skin and horns) had to turn to trading goods between the dimensions to make a living. Now, after untold generations of trading, they are the shrewdest traders in all the dimensions, and their home dimension is the largest Bazaar ever. Here you can buy devices both magical and technological, for a mere fraction of your body weight in arms and legs...
Skeeve quickly falls in with a mixed crowd of Imps, Deveels, Perverts (Sorry, Pervects!), Trolls, unicorns, and a young dragon. He undertakes a number of adventures, most of them both magical and dangerous.
The first four books are great. The rest are also great, but after the fourth book Robert Asprin starts slipping into a 'Skeeve is confused, and here's how's he's feeling and dealing with his problems, emotionally' theme. It's not quite so comic, or adventurous, and it can really get on your nerves at times. Even so, if you read the first four (and you should), you'll probably read the others, and not regret it.
Most of these books are told from the first person perspective of Skeeve, but those books with 'M.Y.T.H.' (as opposed to 'Myth') in their titles are told by one of the other of his companions. There are some minor changes to the style and mood series every once and a while, as Robert Asprin kept stopping work on the Myth books due to side projects or writer's block. It took 23 years to pump out the 12 books he had been contracted to write.