New web-based book by Matthew Butterick. From his opening chapter:
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.