In SmartGo Books you can replay moves in diagrams, solve problems interactively, and export board positions. This means you need much more than just images of diagrams: you need a way to represent all the Go positions and moves as well as the text.
At first, I was using SGF with some extensions to specify book text and Go data. However, it soon became obvious that the tree structure of SGF and the linear structure of a book were not a good match. I ended up creating a new file format to simplify the creation, review, and display of digital Go books. It still takes a lot of work to convert printed Go books to digital form, but this format helps.
You can download the gobook format description (PDF). While this is work in progress, it has proven its value in over 70 Go books, representing a wide variety of challenges, from the Elementary Go Series to Invincible. I hope other people will find this format useful.
Tools for this file format are still pretty limited; for now, you can open files in this gobook format in the upcoming version of SmartGo Kifu. And the Mac version of SmartGo that I’m working on knows how to handle this file format, of course, but that’s for another time.