If you're thinking of writing to an author, try this:

"I just wanted to say that I think your writing is hilarious.
I went to Powell's yesterday with my 11 year old nieces and we sat and
read The Book of Fun for almost an hour - and then we bought it and continued
to read it! . . . Thanks for being so entertaining . . . keep up the good work!

 PS - I never have written to an author before, but you are so
funny that I just had to!"

Thank you VERY much. It's messages (and purchases!) like these that keep me going. :)