Use UIOP [1] for all path/filesystem (and more) operations. It's part of ASDF which means it's bundled with every Common Lisp implementation that counts. Generally, if something is covered by UIOP, you should use it instead of depending on a different library.

Binary parsing depends on the application constraints.

You can't go wrong by studying Frode's binary-types (not the same as PCL binary-types) though [2].

[1] https://common-lisp.net/project/asdf/uiop.html

[2] https://github.com/frodef/binary-types/