I believe there is no single source that someone can read and master it. Probably the best learning path is to start with Tutorial and bird's eye view - Jujutsu docs, its linked tutorials and jj init — Sympolymathesy, by Chris Krycho.
The command line jj help is awesome and works for subtopics (e.g. jj help git and jj help git clone).
After using it for a while, search for jj tug alias—it is used almost by everybody to mark automatically the changes to push—and read stupid jj tricks or watch his video.