Spindle's End, by Robin McKinley. McKinley is one of my favorites for sheer get-lost-in-it stories. I read her when I want to get away from things for a bit. Spindle is one of my favorites. It's a retelling of Sleeping Beauty, and completely engrossing.
I think you can probably tell a lot about my mood by what I choose to read any given week. I think the more relaxed I am the more I go in for classics or non-fiction. When I start burying my nose in the (digital) pages of fantasy land, I tend to be more stressed.