A shell book is the basis of another book. That is, the shell book sets the page layout, locations for text box locations and pictures, and so on. These are not language-specific.
As mentioned in Bloom overview, shell books contain words. These words are the source text. When a book is made from a shell book, the user can translate that source text into another language.
You can use Bloom features to create shell books for others to use.
Essentially, each Bloom book is a little database of texts in various translations, ready to be displayed either in the prompting box, or in the text itself. An interesting implication of this design is that any local language book can become a shell book for other languages; the formats of a shell book and local language book are identical.
In
Collections, Sample Shells are below Sources For New Books. These were installed with Bloom.
https://vimeo.com/album/3146448/video/114024308 is a video to help you learn about templates and shell books.