Book details in a reading tracker app

Your team is building a reading tracker app. Users add each book they read by scanning its barcode or typing its ISBN, the book's 13-digit number. The app then shows the book's title, author, cover and page count. Those details come from a book data company's API, which returns the cover as a link to a 50 KB image.

The API charges $0.004 a call and takes about 600 ms to answer. Your contract allows 20 calls a second.

You expect 2 million users. They will add about 400,000 books a day, at most 10 a second in the evening. Shelves and book pages will show book details about 30 million times a day. A book's details rarely change once it is out, but a publisher sometimes replaces a cover.

What happens between a scan and the book showing on screen, and what do you store along the way?