Books, old purchases not available for download or anywhere.

I had an ipad around 12 years ago and used to read often on it, over time I must have bought around 80 books or so, but stopped using it. I have now bought a new one and figured I'd re-read some of those books since it's been so long, but most of them no longer appear in my library available for download, only 25 appear. They all appear as purchases if I check my history but are no where else to be found.

How can I download them again and why are they not easily accessible?