Z80N instruction set - complete reference and Table form based on ClrHome.org table, with Z80N extensions
"Dev Tuesday" - small snippets of Z80N assembly
Patricia’s Spectrum Next Links - collection of useful articles/links maintained by Patricia Curtis (pro-game-developer since 8-bit micros times)
Simon N Goodwin's collection - pieces of code and knowledge, focused on on-machine development using the Zeus assembler (by Simon Brattel and Neil Mottershead). The (native) Zeus is available in the NextZXOS distribution thanks to the courtesy of Simon Brattel, and updated with few new features to be usable for Next-specific development by Simon N.
Tomaz's ZX Spectrum Next Assembly Developer Guide is almost like "User manual" for Assembly developers. PDF is available for download for free and you can also buy a printed book.
Classic-Z80 (not Z80N, but still mostly relevant) 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://ped.7gods.org/z80.pdf - Darryl Sloan - First Steps in Z80Assembly Language new
http://map.grauw.nl/sources/external/z80bits.html - various small routines
https://www.msx.org/wiki/Assembler_for_Dummies_(Z80) - excellent MSX-computer fan site - has also wiki with articles about Z80 programming