Missing book title

[pussycat37]

I gave the book to someone and never received it back: It's about a man who works in a psychology (?) department of a university. On the campus site, there is a tree and from somewhere near this tree he gets in touch with another reality where he falls in love and tries to help social problems (of which I don't remember any detail).
It's one of the most wonderful books I've ever come across.
Does anyone know what the title could be?
Thanks ever so much in advance!