Matthew Butterick has unveiled his new web book, 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.
It's an astounding piece of work: a comprehensive, no-nonsense, gorgeous handbook for anyone interested in typography. I took my first peek at it the other night and didn't go to sleep for about two hours.
The book is free to read and thus relies solely on reader support, of which there are many options.