I agree with Jess, all the books in the Wardstone Chronicles have been thoroughly enjoyable and even as someone in my late teens :P I still found myself immersed throughout.
My one criticism of the books are the endings. Though it was clear the first book was to be continued, I felt a little disappointed by the endings of the other three books; being able to pick a few holes in the plot and such. However they are "children's" books so I'll accept that as an excuse seeing as the rest of the stories are so fantastic!
I wouldn't recommend them as adult books at all, seeing as I have bothered relatives in to trying to get into the books - to no avail. They are however, as I have said, very enjoyable and easy to get immersed into if you have half an imagination.
I've just pre-ordered the 5th book!