This is a misnomer in a way. A book is not 'shared,' a book is pointed to. Sharing suggest that 'Pathagoras' owns the rights to the book and controls who can share it's content. That is not how we have designed the program.
Let's step back to the definition of a book: A book is a pointer to a Windows/Word folder. Nothing more, nothing less. A user who wishes to add a book to his or her library simply needs to point to the location where the documents are filed. So, to 'share' a Windowns/Word folder, you simply must put it on a drive where others can get ('point') to it.
So make sure that the folder that contains the 'common' content has been saved to a common location. It typically is a folder on the network server, but it can just as easily be a Dropbox, NetDocuments, OneDrive, etc., folder. There should only be one such folder. (That is, don't keep the 'originals' on your computer and constantly copy updated content to the 'common' location. Just keep the original files in the common location. That way, changes you make will be immediately available, and you won't have to worry about forgetting to copy them. (Of course, you should keep backup copies, but that's a security and safety issue independent of Pathagoras.) Just tell the 'sharee' to set his or her book to the common location.