One of my favorite books on design is Thinking with type: A Critical Guide for Designers, Writers, Editors, & Students. This book helped me a lot when I was a junior designer and I'm still using many of the principles I've learned to this day.
It's an easy way to start learning about typography
Typography can be a daunting subject! I remember as a junior designer I was deathly afraid of being found out that I didn't know as much about typography as I should have. This book taught me the basics of typography in a friendly and visual way that helped me learn without fear.
It tells a visual story of typography
Many nuances of typography come from how certain type was created and used before the age of computers. This book outlines the history in a visual way that makes it really easy to understand why certain types are used in certain ways and why they evoke certain emotions.
It outlines principles that are practical
Aside from all the histories and fun facts, this book outlines key principles of typography that can be immediately put to use whether you're designing for print or digital products. From the grid, usage of drop cap to avoiding the lone word, I was able to put these principles to use the next day I went to work. And I sounded a lot smarter (I think)!
Among all the design books that I've owned, this book is the first one I'd recommend to a fellow designer. Is there a design book that's your all-time favorite?