Easy to do! I just piled them one on top of another, you can arrange them vertically or stagger them for a wider look. They can be attached permanently or you can use some kind of clamps to attach them to each other. I used vintage clamps for a rustic look.
This link tells you exactly how to do it:
For stability, attach them together and anchor them to the wall if you have small kids.
Here is a tutorial: https://www.sewmuchado.com/2014/08/diy-wood-crate-bookshelf.html
Hi! Maybe this will help
Not for books but this is how I used crates for storage over a toilet in a vey small area.
This might help: