These are great questions with complex answers...
In terms of print books:
Leanpub is a self-publishing platform only sells ebooks. However, Leanpub authors are free to produce print books based on their Leanpub ebooks, and sell them wherever they wish. Some do; many don't.
In terms of what 75% done means:
If an author says that a book is 75% done, all this means is that at the time they last published, that number is how complete they think the book is. For some authors, 75% done might mean 3/4 of the planned chapters are written; for others, it might mean they expect the proofreading and minor edits to take 25% of the time. We encourage authors to make the % done to refer to the amount of content, but we have no knowledge of what a particular author means by 75% done.
Most Leanpub books have an Email the Author button on their landing page which takes you to a form where you can email the author questions like this.
Finally, all Leanpub books have a 45-day refund policy. So, you can buy a book from Leanpub, either in-progress or completed, and if you are unsatisfied in that 45-day period (e.g. if the book is incomplete and the author is not making any progress), you can click a button to get a full refund...