Many Leanpub books are published serially or in-progress; that is Leanpub's specialty.
If a book is marked as only X% complete by an author, that means the author has not completed the book.
If you want to ask the author about their publication schedule, you can find an "Email the Author(s)" link on the book's landing page, unless the author has disabled it, which is rare, because authors who publish in-progress books are usually welcome to feedback from early readers.
Here is a free resource that explains our in-progress publishing philosophy: