Useful Zilog Z80 coding and optimization resources:

Z80 ASM Optimization Tricks - v2.1 (by Adam Ziemba, 2006)

Z80 Optimization Technies v1.1 (by Christopher Tremblay, 2000),

Z80 instruction set

Twitter bot for BASIC and Assembler programming online:

How to Write Spectrum Games v1.1 (by Jonathan Cauldwell)

First Steps in Z80 Assembly Language v1.2.4 (by Darryl Sloan) + YouTube (4 videos)

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