The app. asks for books list from the cloud, for each book that does not exist in the app's bookshelf, it asks for details such as (book name, cover image, author, publisher, description (blurb) etc.).
The books are displayed on the app's bookshelf with a cloud icon on each book. This means the book is available but was not yet acutally downloaded from the cloud.
Afer getting the booklist, it starts downloading 5 books that where purchased lately.
You can for download of a specific book by opening it for reading or long click on the book and selecting "Download" (right, bottom corner).