Beautiful Web Books
This new book on typography gives a pretty great run-down on the basics, and beyond, on using text. Butterick does seem to push a little bit too hard on using his own fonts, so although the book is free, it feels like a big ad. The information is still very useful and sound.
Check out Practical Typography, by Matthew Butterick.
Another free web book published this year is The Shape of Design by Frank Chimero. It also manages to be beautiful, while living inside a web browser. It has no underlying sales message either.