I'm not sure which behavior is more desirable.
Currently, the book creator uses the jStorage api to save data into the browser's localstore as a collection is being created. Choosing the "Save and share your book" synchronizes to a new article in the "Book:" or "User/Book:" namespace. There is a corresponding "Open in book creator" action which repopulates local storage using the contents of these server-side articles.
Problem 1) There is no UI text indicating that you are saving to local storage and not a server. If you connect using another browser or computer your work will have disappeared, possibly causing the work to be repeated.
Problem 2) This makes it much more difficult to collaborate on a collection. It breaks any built-in edit conflict resolution. It is a totally different workflow than editing a wiki page, although the eventual storage is in fact a wiki page.
Problem 3) Only one page can be in draft at any given time.
The alternative I'm proposing is that the collection is stored as a Sandbox page (or a private page, if such a thing exists) until you choose to "Save and share". Book creator edits can be made through a new Extension:Collection api, which would simply add or remove the line from the book's wikitext source. Or, changes (eg, adding three pages) could be batched, previewed and diff'ed, then pushed to the server when the editor is satisfied.