Today’s software tools have weird names. We call a “library” some collection of functions that you can use in your program.
I think that software repositories (where apt downloads your programs from) should be the actual libraries, since that’s where you go to get your information; Meanwhile individual packages of information should be called books because they are one solid object containing a bundle of information.
The metapor is that the individual package is a library of exported functions (the books). Package managers are more like inter-library loans in this metaphor, but I think that is pushing the analogy further than is useful.