Hugh Lofting wrote twelve Doctor Dolittle books: ten were fully realized novels published during his lifetime, while the other two were posthumously- released story collections combining previously unpublished material with stories that had appeared in print decades before (compiled and edited by the author's sister-in-law, Olga Fricker).
In addition, when in the 1930's he was not producing new Dolittle books with the one-a-year frequency of his early career, Lofting pacified fans with a pair of books that one might say are on the Puddleby periphery -- Dolittle-related, without actually containing any stories about the Doctor. Specifically, Gub-Gub's Book is a small collection of tales in which the animals appear without their veterinary companion; and Doctor Dolittle's Birthday Book is a little day-book illustrated with pictures and quotations from the stories. Counting these two volumes, we have a grand total of fourteen books in the "Dolittle family."
Below you will find two lists. The first gives all fourteen of the books in publication order, while the second lists the ten Dolittle novels (i.e., omitting Gub-Gub's Book, the Birthday Book, and the two posthumous story collections) in order of their internal chronology (since, for instance, the events of the third-published novel are actually supposed to have taken place before those in the second-published novel, etc.).
The Dolittle Books, in Publication Order
The Dolittle Novels, in Order of Internal Chronology