This sounds intentional, as I recall its not supposed to let you open the GUI when clicking any book slot unless you are sneaking, regardless of if you have items in your hand or not.

If you want to open the GUI regardless of holding an item or sneaking, just click the wood instead of a book slot.

Closing for lack of response. From testing in game, all interactions felt as expected, as the bookshelf normally does not open the UI if you have anything in your hand when sneaking, vanilla passes the logic to the item.