Dr. Seuss is probably one of the most famous children's writers to have lived, so it should come as no surprise that when someone discovers an unpublished Seuss, it's going to get published. Random House has found some short stories written for magazines that Seuss penned in the 1950's, and are re-releasing them under the title, The Bippolo Seed and Other Lost Stories, coming out this fall.
There are a few other books of his, based on his notes and rough sketches that were published posthumously: Daisy-Head Mayzie, Hooray for Diffendoofer Day! (completed by Jack Prelutsky and Lane Smith), and My Many Colored Days (illustrated by Steve Johnson and Lou Fancher). While they are all enjoyable books in their own right, I don't think they compare to Seuss's earlier works. And don't get me started about the Cat in the Hat Learning Library series, mostly written by Tish Rabe and usually illustrated by Aristides Ruiz. They may rhyme and look like Seuss's style, but they lack Seuss's genius for mad invention and wackiness. What I've seen of the cover and a few of the illustrations for the Bippolo Seed look terrific -- I think this book will "feel" more like the classic Seuss.