I am puzzled.
When in BookSmart you do a File.... Export.... Export Book Project the resultant file has a type of .bookexport, not .book, so you should have a file named mavis at 80.bookexport.
The mavis at 80.book is the master file of the collection of files that make up your book and is found in the BookSmartData folder (under sub folder mavis at 80). It is not the complete book and is not openable by associating the file type with BookSmart. It can only be used by BookSmart when you do a File....Open.