Dr. Seuss (his actual name is Theodor Seuss Geisel) is one of the most popular children's book writers in the US today. His first novel was And to Think That I Saw It on Mulberry Street, which was written pre-World War II. Since they he has written dozens of children books of all varieties, and is a staple in most children's homes as a first book to read.

So, what was Dr.Seuss's last book? Well, technically there are two correct answers. The last book that he made completely by himself, illustrations and everything, was Oh, The Places You'll Go! (1990).This book is a popular choice for high school and college graduates, as it has many parts that apply to graduation. Unfortunately, Theodor Geisel (Dr. Seuss) died less than a year after the release of this book, so he never got to fully enjoy his last book's impact. To date, Dr.Seuss remains one of the most popular authors of all time. HIs book Green Eggs and Ham is one of the five most popular books of all time in the United States.

However, there were a few books made after Dr.Seuss's death. These were mostly based off of unpublished scripts that Dr.Seuss had, or ones that he had simply failed to fully illustrate. They were released to the market in quick succession in the early nineties, until the last one in 1995, called Hooray for Diffendoofer Day!

Thus, in answer to what was Dr.Seuess's last book, it is all opinion.  Whichever one of these books you choose, you can't be considered wrong.