I just got an email from Amazon informing me that they have retired Kindle Textbook Creator in favor of Kindle Create.
Kindle Textbook Creator is now part of Kindle Create. This is a part of our ongoing effort to simplify tool choices for authors so that any type of book can be created using a single tool. Thousands of authors have used Kindle Create to prepare Print Replica textbooks with the same ease of use that Kindle Textbook Creator offered.
We recommend that you switch to Kindle Create for your next Print Replica eBook. Kindle Create offers the same features as Kindle Textbook Creator and will continue receiving software updates. Files created in Kindle Textbook Creator (.kcb format) can be opened in Kindle Create, so you can seamlessly transition to the latest software.
For those not familiar with these apps, Kindle Textbook Creator was Amazon's original tool for making the PDF-ish Kindle ebook format known as Kindle Print Replica while Kindle Create was developed to make reflowable Kindle ebooks.
That was a while ago, however. Kindle Create has been capable of making KPR format ebooks for several years now. In fact, Kindle Create is what I used to make Kindle editions of the two workbooks I have in the Kindle Store.
Kindle Create is Amazon's preferred tool for producing books to be sold on its site, and this includes print books as well. A recent update added the option to format paperback books for KDP Print.