Merry Go Round in Oz, published in 1963, was the last of the Oz books published by Reilly & Lee, and the only one published in my lifetime - although I was too young to know anything about it! This was written by the mother and daughter team of Eloise and Lauren McGraw, and illustrated by Dick Martin - his first full length Oz book. In 1989, Dick Martin designed a new cover for the first reprint edition of this book.
By this point in the series, the variety of authors and illustrators involved in creating the books makes the late sequels feel a good deal less cohesive to me. I'll admit to preferring the earlier books, but it's still amazing to see what a long run the original book inspired. This title is the end of the forty book "Oz Canon", the run of books considered by many to be the Oz series. Quite a few titles have been written since, although relatively few have had widespread distribution.
The International Wizard of Oz Club published a second Oz book by the McGraws in 1980, titled The Forbidden Fountain of Oz, again with illustrations by Dick Martin. In 2001, Hungry Tiger Press published The Rundlestone of Oz, which was Eloise McGraw's last book. This was illustrated by Eric Shanower.