The following are links to sites that are useful if you are learning to develop addins.
C program notes, dated from 1995 but very clear and at simple
level explaining the syntax you'll need to know.
Home of the z80, massive site with many links to Z80 CPU resources
The following are links to other addin developers sites...I don't intend to try to rate or order them, just list them:
Bertus Beer Source code
Dimitriy Source code
John del Ferro Source code
Chris Harris Source code
Mr. Idogawa Source code
Gerd Krahmann Source code
David Riley (Irish_iiii) Source code
Shai Seger Source code
Daniel Schmidt Source code
Marcel Timm (Mantillo)
Link to rex technical sites other than addins developers