Recent knowledge from buying a pennywhistle and checking out books for a young teenage girl who is Riverdance mad:

Stay away from the Cathal McConnell CD book (though the longer tapes may be good). Nice to listen to, but he immediately goes beyond the capacity of the beginner. The Clarke book and tapes come recommended. They come in a box. The Clarkes cost about 15 bucks U.S. (I forget how much the box costs, not much more). You can buy others for about the same -- there are internet sites that talk about handmade whistles, but that seems daft to me. I men, the whole point is cheap, right?

The Clarke pennywhistles have a different sound than others, because they have a wooden fipple (the mouthpiece), which makes them breathier -- but less shrill than the plastic ones. You need to make sure that if you are buying one for starters that you are getting a "D" flute and not a "C" -- the "C" is the original, but most people play "D" flutes (Clarke's lesson tapes have been redesigned recently for D flutes). I guess people graduate to "C" flutes.
yours, Peter T.