I agree with Dan, Munari's "Design as Art" has an inspiring collection of essays I keep going back to.
I always find Robin Kinross's books incredibly interesting as well. Even though they aren't entirely about the practice of design, both "Modern typography" and "Unjustified texts" are invaluable resources. Actually, anything published by Hyphen Press is worth a read—they've also published quite a few of Jost Hochuli's books on typography and book design.
I agree with Dan, Munari's "Design as Art" has an inspiring collection of essays I keep going back to.
I always find Robin Kinross's books incredibly interesting as well. Even though they aren't entirely about the practice of design, both "Modern typography" and "Unjustified texts" are invaluable resources. Actually, anything published by Hyphen Press is worth a read—they've also published quite a few of Jost Hochuli's books on typography and book design.