What kind of book is this?

I'm getting ready to write my sermon for Sunday October 3, and I could use a little help.  In chapter 20, John (or whoever actually wrote this book) talks about why the Gospel was written.  So what do you think?  What kind of book is it?  Is it science?  A textbook?  History?  A fairy tale?  Why do we still read it 2,000 years after it was written?  What do you think?