What is Leanpub? I'm asking as a reader.

Leanpub is the combination of two things: a book writing workflow and an online bookstore.

Leanpub provides authors and publishers with the best way in the world to write, publish and sell in-progress and completed ebooks. When you buy a Leanpub book, you will get all future updates to the book for free. Updates may include simple improvements like typo fixes, or entirely new chapters, depending on how complete a book is when you buy it. All our books are DRM-free.