Typography in ten minutes — Butterick’s Practical Typography
This is a bold claim, but i stand behind it: if you learn and follow these five typography rules, you will be a better typographer than 95% of professional writers and 70% of professional designers. (The rest of this book will raise you to the 99th percentile in both categories.)
All it takes is ten minutes—five minutes to read these rules once, then five minutes to read them again.
See also the Summary of key rules and the full Butterick’s Practical Typography web-based book.