A small list of important and useful Typography and Design books.
The Elements of Typographic Style by Robert Bringhurst
Thinking with Type by Ellen Lupton
Detail In Typography by Jost Hochuli
Stop Stealing Sheep & Find Out How Type Works by Erik Spiekermann
Type and Typography by Phil Baines
The Typographic Desk Reference by Theodore Rosendorf
Also take a look at this extensive list of design-related books over at Speak Up.