I found out about this book by watching a video reading/parody about it.
http://www.youtube.com/watch?v=ABLqNNBW_oQ&feature=youtube_gdata_player
I must say, it's changed the way I think about things. The hat must be a metaphor for all the things we take for granted in life. Once you notice something is gone you begin to appreciate it even more, and thus the epic search commenses. The characters all represent the shortcomings in life and how things don't go as planned on the first try, but with persistence you will achieve your goals.
In other words it's a book about hats.