Any suggestions for web resources on Robinson Arithmetic, PA, primitive recursive functions, etc.?

As I’ve said, I’m in the middle of revising my much-downloaded introductory notes Gödel Without (Too Many) Tears. I’d rather like to add to the end of each chunk of the notes a very short section of “Further/Parallel Reading”, where this ideally points to again to freely available material — i.e. webpages or pdfs which are at a similar sort of introductory level (and very clear, and relatively short).

I’d love to hear, then, about any free resources out there (other than Wikipedia!) that you have found particularly useful as a student or teacher, on any of the following topics from the first half of the notes:

 The very idea of a diagonal argument
 Robinson Arithmetic
Induction
(First-order) Peano Arithmetic
The beginnings of the arithmetical hierarchy/quantifier complexity
Primitive recursive functions
Why the p.r. functions can be expressed in the language of basic arithmetic/ represented in Robinson Arithmetic.

Pointers to other people’s lecture handouts and all other suggestions most gratefully received!