The Laura Ingalls Wilder books, particularly The Long Winter, Little Town on the Prairie and These Happy Golden Years. I just love them whenever I feel low. I love the warm family relationships, and Pa and his fiddle.
As previously mentioned, the James Herriot books are lovely.
When tired and sad, I also read a lot of Bill Bryson. Sometimes I wish he would adopt me (even though I am middle aged and perfectly capable).
Agatha Christie is always a good read when life is crap.