If you like Groundhog Day that much, you should check out the novel it was loosely inspired by, The Strange Life of Ivan Osokin by P.D. Ouspensky. Ouspensky isn't known as a novelist (he became a guru type) but up until the end where it gets a bit doctrinal, I thought it was a thoroughly delightful book. It's also a psychological tour de force, but I don't want to spoil the story by explaining why.

Oops: apparently not inspired by but, let's say, resonant with. See the (rather pretentious) blurb by Harold Ramis on the back of the book:

http://books.google.com/books?id=d15Nk9MmAMcC&pg=PT1&...

Bill Murray's charachter is seen as a bit of a philosophical parable about self-improvement. See http://www.schindler.org/psacot/20010813_ghd.shtml

There's so much room for interpretation in this movie it's hard to know where to start. Consider, for instance, the fact that Murray's character apparently spends decades in this state.