Butterick’s Practical Typography
Typographer Matthew Butterick, who helped design fonts for Apple, Microsoft, and others, has released an exceptional online book, called “Butterick’s Practical Typography,” that is a must-read for anyone who works with text. The book revolves around a few simple rules that will dramatically improve your understanding and implementation of typography. While the book can be read for free, you can pay him back by purchasing his fonts or his “Typography for Lawyers” book, making a donation, or just telling people about the site.
Anyone interested if text formatting should read this online book, and especially the section titled "how to pay for this book".
Do it now. You won't regret it.