Z80 programming
From SpecNext official Wiki
"Dev Tuesday" - small snippets of Z80N assembly
Classic Z80 tips and tricks and other links:
http://www.zilog.com/docs/z80/um0080.pdf - Official Z80 documentation
http://www.z80.info/zaks.html - Rodnay Zaks - Programming the Z80
http://map.grauw.nl/sources/external/z80bits.html - various small routines, mostly-relevant to Z80N
https://www.msx.org/wiki/Assembler_for_Dummies_(Z80) - excellent MSX-computer fan site - has also wiki with articles about Z80 programming, mostly-relevant to Z80N