If you're the fan of an author or publisher with a Facebook Bookstore, you can click the "Shop Now" button or link pinned to the top of their Facebook Page for a list of available books. From there you just license an ebook and start reading all within the Facebook experience.
After you've licensed an ebook on the Facebook Bookstore you can start reading it immediately by clicking the green Read button. If you want to read later, you can simply return to the Facebook Bookstore, find the book, click on the green Read button, and continue reading from where you left off.
If you forget how you got to the Facebook Bookstore, you can get there through most participating authors' and publishers' Facebook pages by clicking on either the Shop Now button on their page or through a Facebook post that's pinned to the top of their feed.
The reader app is automatically launched within your Facebook session when you click the green Read button, so you don't need to download a separate reader application.
You can read your ebook anywhere that you use Facebook - smartphones, tablets, PCs, and Macs.
All the ebooks are stored "in the cloud," and the reader app fetches portions of the ebook as you read them. So there's no ebook file that you need to download.