Movable shelving is every librarian's dream. It works under the following assumption: books that are requested to be pulled from the library stacks are done so one at a time. It therefore follows that only one aisle, the aisle the requested book is in, is needed at any given moment. Therefore more books can be stored in any given place by keeping them compressed and just moving the aisle around.
Sound like some strange 6th dimensional fairy tale? Don't be too hasty.
In their most primitive form, movable shelves are human powered - cranks at the end of each stack wheel the shelves left or right and, in the process, move all the shelves in the line creating a aisle. Clamps on each stack would lock them in place to reduce the risk of injury.
The cream of the crop are computer controlled monstrosities with keys to prevent unauthorized access. Because these particular versions are motor-driven, the risk of getting crushed by some other unsuspecting page is much greater than with the lower-tech versions. To combat this, pressure sensitive floor panels are generally installed that (as well as controlling the aisle's lighting) also act as an emergency cut-off switch to keep the shelves from moving if they detect anyone in the aisles.
The only problem with that system is in libraries with incredibly high stacks - these movable systems negate the possibility of movable ladders, so some shelf climbing is necessary. If you're clambering up a 20-foot high bookshelf, your feet aren't in contact with the floor and...
To combat this, pages (hopefully!) bring their library carts into the aisle with them to keep the floor weighted down.