Just in case you missed it, last May School Library Journal started reviewing iPad/iPhone apps. Here's the link to their introductory article about it:
Meanwhile, based on some of those reviews, I have just downloaded three classic children's books to my iPad:
Go Away Big Green Monster, The Monster at the End of this Book, and Harold and the Purple Crayon.
Of the three, Harold is the clear winner, with lots of "hidden" interactivity and the opportunity to fill in the gray lines with satisfying purple crayon. That's what makes the story move forward, you "helping" Harold to draw his way through the adventure. The "lite" version is free, but it's also very "lite" indeed, with all the interaction and none of the story. It's worth the money to pay for the full version, where touching on a distant bird lets you zoom in to see a mother bird feeding her babies. Touching
The Monster at the End of this Book is also surprisingly successful, especially when you get to break Grover's knots and smash down his brick wall with a mere touch of the finger. The metaphor works mostly... except if you sit there long enough, Grover, who has been asking you not to turn the pages, will in fact say "oh, get on with it, turn the page." Probably an important moment to differentiate the real world from fiction for your average young viewer.
Go Away Big Green Monster works a whole lot like the die-cut paper version, with added sound effects when various face-parts (eyes, nose, teeth, ears, squiggly purple hair...) appear at the touch of the finger. And then disappear, as promised, also by the touch of a finger. Definitely fun stuff for the preschool set.
And, as per that article, I'm looking into two more sources of reviews:
Commonsense Media and Children's Technology Review