For the bookshelf, I also needed some books of course to make it look like it has been used a lot before. So I created this book and scrambled it throughout the bookshelf randomly as you can see in the image below. Then I combined all the books in groups that were randomly spread out (see the image below with the lit-up books as one group).
For each of these groups, I would make an individual texture and assign it to the group. Below this text, you can see individual textures I made for the groups. Also for these, I found a flat image online and used this to create the texture by image tracing it and implementing it in Substance Painter. Then I also gave the books a leather feel to it and some scratches.