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The Elements of Typographic Style, by Robert Bringhurst (Hartley & Marks, Vancouver, 1996). Typography for the book, from a design perspective rather than a typesetting one. A thorough and thoughtful treatment of typographic aesthetics.
Stop Stealing Sheep & Find Out How Type Works, 2nd ed., by Erik Spiekermann and E.M. Ginger (Peachpit Press, Berkeley, Calif., 2002). A fresh and engaging look at how and why type works, by a leading graphic and typeface designer. A clever, insightful, and eye-opening tour de force of the typographic arts.