In Skyrim, the bookshelves in the houses were a type of container. So, you'd open the each shelf of the bookcase and fill it with books. There were a certain number that would fit on the shelf, depending on its size. Then you could click on a book and read it.
It would be great if they implemented that functionality in ESO. Where we can store books in bookshelves, so they're essentially in storage and not count towards the item limit. But, you could still access and read them from the bookshelf.
or they could count as the same type of furnishing as undaunted busts/trophies. That way we could at least save a hundred slots or so if we want to do a library house.
A suggestion would be to have a row of books type of furnishing which contains all the books in a collection, and this could be bought from the mages guild. Thus you can place the rows of books in empty bookcases!