Useful Zilog Z80 coding and optimization resources:

Z80 ASM Optimization Tricks - v2.1 (by Adam Ziemba, 2006) https://www.ticalc.org/archives/files/fileinfo/285/28502.html

Z80 Optimization Technies v1.1 (by Christopher Tremblay, 2000),  https://www.ticalc.org/archives/files/fileinfo/108/10821.html

Z80 instruction set https://clrhome.org/table/

Twitter bot for BASIC and Assembler programming online: https://twitter.com/ZxSpectrumBot1

How to Write Spectrum Games v1.1 (by Jonathan Cauldwell) https://jonathan-cauldwell.itch.io/how-to-write-spectrum-games

First Steps in Z80 Assembly Language v1.2.4 (by Darryl Sloan) http://ped.7gods.org/z80.pdf + YouTube (4 videos)

The ZX-Spectrum screen layout: part 1 + part 2 + part 3