Pouet's Tiny Intro Toolbox Thread was one of sizecoding.org's main inspirations.
The Hugi demoscene diskmag has been kind to the sizecoding movement. They included many programming articles since it started in 1996. Additionally, the Hugi Size Coding Competition Series is the gold standard for learning how to sizecode: Each competition was given a specific task, then participants submitted their results. (All of the entries with their source code are available, as are the rules for each challenge.) Hugi #35 also provided a 256b intros round-table between Baze, Digimind, Řrřola, Pirx, Optimus, and Seven.
During the last years 256b intro coding was also a topic in some demo party presentation sessions. The recordings are available on YouTube. There are part 1 and part 2 of Řrřola's talk "Magic of 256-byte x86 intros" held at Demobit 2017 and 2018 respectively. A similar talk "256 byte intros on modern PCs" was held by TomCat/Abaddon at the Function 2017 party (mostly in English with parts spoken in Hungarian). Those presentations cover everything from setting up the coding environment to all kinds of code optimization, constant reuse, flag tricks, palette generation, and how to get from an idea to the final intro.
fysnet.net has an extensive list of DOS .COM register starting values. If your environment isn't listed, a program is also available that can print out all of the register starting values. See also DOS Register Starting Values to understand why the register starting values for .COM files are set the way that they are.
The Sparks And Flames x86 Instruction Chart is awesome, once you can "read" it. Find out what the bright green (1-byte) commands do, and use them. For the extreme sizecoding (<=32 bytes) it's also very useful to have the HEX value in sight.
Rene Jeschke's preservation of siyobik.com's x86 Documentation is one of the most compact and understandable references to the x86 instruction set you'll ever see.
Intel IA-32 Software Developer's Manual Instruction Set Reference hypertext version.
Robert Collins' Undocumented Opcodes page has some gems you can exploit depending on your target environment.
PC Speaker Basics For everything from simple bleeps to sophisticated synthesizers.
Program Segment Prefix : what's in front of our .COM code, and why.
Never underestimate The Hidden Power of BCD Instructions.
An optimization guide for x86 platforms By Agner Fog Chapter 10: Optimizing for size.
Code Gems by Ervin/AbaddoN Nice collection based on the Imphobia diskmagazine article.
BIOS data area: Direct system communication overview. There are environment values here you may be able to utilize.
Summary of MIDI messages : Summary of all basic MIDI messages.
Roland GS/GM2 Drum Set List and Note Map : Detailed Description of all MIDI drumkits.
Online IEEE 754 Converter Excellent tool for optimizing/reusing FPU constants
Tinyprogs are so small that you sometimes don't need the source code to understand them: Just pop them into a disassembler (like IDA) or a debugger and look at them. If you want to grab specimens for study, here are some places to start.
HardCode is a repository of 7800 64KB and smaller intros.
256b.com is a website dedicated primarily to 256b and smaller releases.