When adding links to other books, you can now select multiple books in the 'Add Link' window:

And the program will now automatically create backlinks (that is, a link back from the book you are linking to). (If you want to disable this, choose Tools->Options->Data Entry, and uncheck "Record links: Add backlink".)

In addition, when you remove a link the program will automatically remove the backlink. (If you want to disable this, choose Tools->Options->Data Entry, and uncheck "Record links: Remove backlink").

And finally, the explorer preview pane in the Book View now shows the book links: