I'm so glad I helped Shelfy live longer xD. Although I noticed a little bug when colliding on the edges of the shelf and the books seem to stretch (idk if you wanted it to look that way but you can disable rotation in certain directions in rigidbody component)
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I sacrificed cleaning up some book issues to have a restart system =)
I am not sure if rotation is the issue (but thank you for the idea, I will check it out), I think it is because I set the shelf as the parent option, so the scaling sometimes effects the child? If I knew I would have fixed it haha.
Thank you for giving Shelfy a long happy life.