Practical Typography
Matthew Butterick
A free, opinionated web book on setting type well, written by a lawyer-turned-typographer.
Butterick gives you the 20% of typography that fixes 80% of amateur work: point size, line spacing, line length, and one good font instead of a system default. Read the "Typography in ten minutes" and "Summary of key rules" first and your documents and screens improve the same day. It is the best on-ramp before a heavier text like Bringhurst.