https://fset.common-lisp.dev/Modern-CL/Top_html/index.html

A book-length description of a library, FSet, that provides functional collections for Lisp. By “functional collections” the author means collections – sets, trees, maps, and others – where update always produces a new collection rather than mutating the original collection in place. This is an attractive property for a language, and one that’s hard to do efficiently. As with Purely functional data structures, it provides a change of emphasis that makes for more referentially transparent code.

The book contains a lot of discussion about the library’s rationale and design, which makes it useful as a reference for others designing large libraries (or new languages, which of course in Lisp can mean much the same thing). There’s an equally important discussion about what’s not provided (for example lazy collections) which is really interesting by giving an example of when to stop with a codebase that could otherwise probably grow indefinitely.