Thank you all for the suggestions and for sharing your own experiences of what you do.

I will create a collection for the books I'm done reading and then I'll remove them from device so they are stored in the cloud.

Secondly, I am going to make some suggestions to amazon on how better to have these books being put to use as it feels like I'm just storing these books for no reason at all and ultimately so much energy is spent in running these cloud systems that it's not worth it. I would like to be able to donate them to a local library.