By project
Something being written that these references are for: a paper, a thesis, a course.
Everything the drheap.org site cites, across all five courses and the articles, so it spans semiconductor physics, separation logic, network protocols and safety-critical failure alike. Membership means some page cites the work, not that the works have a subject in common.
What lectures cites.
Sources for a project on arithmetic without Peano's axioms: Grassmann's 1861 Lehrbuch which preceded them, commentary on Peano and his school rather than his own papers, the consistency literature, and the parallel-postulate and non-Euclidean writings as the older case of doing without an axiom. Membership is by citation, so a work here is one the project uses and not necessarily one it agrees with.