The Wheel of the Year is a contemporary fantasy series.
That's another—and much shorter—way of saying that it is an expanding series of connected mystery-suspense novels that are set in present day and involve characters and elements taken from European folklore.
Think Celtic and Viking legends, magical potions, wizardry and the like.
Although young adults might enjoy reading this series as much as the next person, the books were (and are being) written for a much wider audience, ages...oh, let's say...16 through 160. To perhaps give a better idea: the main characters are, at minimum, in their thirties.
The books are (and will be) connected, but each can (and will be able to) stand alone.
As is required with mystery-suspense, there is both mystery and suspense. There is also mayhem, adventure, and tension of a romantic nature. There are light moments, and dark moments. Locations vary, but a good number of events take place in the fictitious town of Granite Springs, Oregon.