I've always enjoyed Robin Williamson's THE PENNY WHISTLE BOOK (Oak Publications, 1977, with pictures of a young Robin in it). But I tend to learn better from hearing tunes, so if that's the same with you, just listen to tunes and let them percolate through your brain down to your fingers.

If jigs & reels & hornpipes seem too fast or complicated, you might start with Morris tunes, which tend to be simpler & go more slowly.

Enjoy! You're going to have a wonderful time. Be sure not to be shy, 'cause you can't play a whistle shyly. (Maybe slyly, though)

Tom on Comfort