I don't remember the title of it offhand, but it was by a local author who published her books through this tiny little bookstore down the road from my house. I never met the author, but apparently she didn't live very far from me. It was a series of books, but they didn't necessarily have to be read in order. I don't think they were written in order either.
They were stories about the different little people who lived in the trees during the different seasons. In the winter they were tiny people who lived in the roots and could only walk on shadows. That was my favorite one, and I remember it vividly. It had a blue cover, and handdrawn illustrations of the people who vaguely resembled humanoid mice.
I have the books somewhere. There was my favorite with the blue cover, one with a yellow cover, and then a green one about the summer people. Too bad I can't remember the name. I must have read them a million times.