I would break that into two challenges: discoverability: knowing that do actually you have a book. this can be ensured by having a recognizable side of each bo

I would break that into two challenges: discoverability: knowing that do actually you have a book. this can be ensured by having a recognizable side of each book visibly exposed. This helps prevent, among others, embarrassing moments when you realize you bought a second copy of a book you already have recoverability: ability to pull a book (for those rare instances that you will actually need to do so) admittedly these constraints would make 3D book packing less dense than a sphere packing. the closest solution is probably mobile shelving [1]. Now we only need to find a way to live in such a place :-) [1] https://en.wikipedia.org/wiki/Mobile_shelving https://en.wikipedia.org/wiki/Mobile_shelving