Photos for free?

I got an email today via Flicker that has me quite conflicted. The email was concerning the photo I've posted here.  The emailer wants to use this shot in a book he's writing about geocaching.  No payment, just a credit in the book.

This is where the conflicted part comes into play.  I know I'm never going to be an Ansel Adams or Galen Rowell kind of photographer.  I enjoy the hobby and I enjoy trying to get better at this.  So even if I think I'm not going to be a full-time professional photographer, I still think I should be compensated for my work if someone wants to use it.

Would the writer of the book just give their book away for free?  Why is he expecting that I just give away my photo for free?  Perhaps I'm overreacting, but this bothers me for some reason.  What do you think?