The Wonderful Wizard of Oz. (I didn't even know that there was a kids' book club at NPR, but apparently they are called the Backseat Bookclub.) I am thinking about reading this book in March along with the book club -- I've always meant to read it, but have never gotten around to doing so.
The cover image shown here, by the way, is from an indie publisher that has released inexpensive (99 cents each) ebook editions of the Oz books with all of the original illustrations, illustrations which -- according to the NPR story -- had a bearing on the book's popularity. While it may seem silly to pay for an ebook that is available for free on Project Gutenberg, bear in mind that they have done a lot of formatting to get the book's illustrations to look right on e-readers. I do have this edition, and it is well worth spending a buck for!