You can make bookshelves usable by sliding a goods lore into the shelf.

First, you build the surrounding bookshelves (preferably another wall behind them) and leave space for two blocks (not where you want it, but the block above and the two above it. In the lower of the two you put a rail, on top of it a goods lorry.

Then destroy the block under the rail, where you want the usable bookshelf. The rail is dropped and the goods trolley falls down.

A bookcase is built over the goods trolley and a piston is built over this bookcase, which is directed downwards.

If you now supply the piston with a redstone signal, it pushes the bookshelf onto the goods trolley.

Now the bookshelf is usable. But the problem is that you can see the goods through the bookshelf. At short distances not always, I have not yet seen through when and when not.

More inconspicuous can make this trap doors that you place in front of the bookshelves. With texture packs that include an opaque version of the trapdoor, this makes it completely invisible.

P.S: This can also be done with "non-bookshelf blocks". In fact, anything you can move with a plunger. So you can hide books in the cracks of bricks, or actually slide notes in a wailing wall that contain your wishes and stuff like that. So you have enough possibilities!