For the pre-school set:
I have given the first four books in the Henry and Mudge series as birthday/Christmas presents at least half-a-dozen times. Cynthia Rylant's series about a boy and his dog is utterly charming and delightful, and was beloved by both of my kids.
The Gift of Fear is not a book that is fun to read, but in my work and in my life I often come across people whom I believe will find it comforting. I highly recommend it, and try to keep a copy on hand to loan/give to people who need to read it.