The big thing I want in a book repository is a hierarchical list which will allow browsing by subject.
One site which does work well for this (for textual books) and which is well-regarded is:
https://onlinebooks.library.upenn.edu/subjects.html
which uses the LoC identifier for a given text.
A project which past me wanted for future me was to read one book from each LoC category, and then to repeat that for sub-categories --- young me did that for the Dewey Decimal system for the high school library, but there were a number of gaps in the collection, and at that time, I didn't have the discipline or resources to make note of them so as to source books from other book repositories.