I read some books cover to cover, and some are kept as references. Books are mainly on Math, Philosophy, and History. There are other topics, too.

I read in 4 languages and GR is very Anglo-centric. That's another issue.

I wanted to track my to-read and read for every book that I found.

I cannot do that with GR anymore.

(I am unsure to whether you are asking about my use case about the sites or the books. So I answered both.)