And, if this is how you actually qualified the path name "\BooksStore.bak", it isn't a valid destination string. Something like 'e:\validpath\BooksStore.bak' should work without issue (or if going over the network- even though that is consider a bad practice, something like '\\servername\validpath\BooksStore.bak').
Edit - If going over the network, be sure permissions are set appropriately.