Books Storage Size

I own an XR with a 128GB plus an additional 50GB in iCloud. I'm currently using 29.2GB iPhone storage and only 21GB of iCloud storage (I know, it appears I'm not utilizing my storage).

I love reading books and right now I tend to read them more on my phone rather than a physical book. I thought when I got my XR I wouldn't have the same problem about storage seeing as I'm barely half my total GB. But now, a pop up showed that i already reached maximum storage and to "manage my iPhone storage in Settings". Can someone tell me what exactly I'm doing wrong? How can I fix this that would allow me to use up my storage properly in iBooks, without the storage issue popping up?

Any help/idea will be appreciated.