I don't have any books on garnishes, plus I'm just an at home cook.. But I do use the site "Fine Dining" for garnish idea's when I want to get more fancy..
I tried to post the URL for you but it seems I'm restricted on that as I have not posted enough on here yet..
If you do a google search for Fine Dining the website should come up for you. Click on one of the three photo's that come up on the home page, and it will take you to more garnishes and how to do them..
I also tried to post a website about fruit and vegetable carving but that was restricted too..
So.. if you google (the art of fruit carving) look for the link..
"Fruit and vegetable carving-ideas and patterns for watermelons."
If this works okay for you, it should be the sixth link down on the google list..
It has quite a few how to do's on carving melons that you might like..
Just thought these might help you out without having to buy books right away..
Cheers!