The 4th edition of Erik Spiekermann’s typography book, Stop Stealing Sheep and Find Out How Type Works, is available free of charge on Google Fonts under a Creative Commons license.
Spiekerman is one of the world’s leading type designers. This is probably the best book on learning the ins and outs of using typography that I’ve found. I bought a copy years ago, but I’m happy he’s released the latest edition for free.
An interview with Spiekermann: Catching up with Erik Spiekermann - Library - Google Design
