This volume is two books bound as one: a new (2008) third edition of The Non-Designer's Design Book and the 2006 second edition of The The Non-Designer's Type Book, 2nd Edition. These are excellent books for non-designers or in fact for anyone who has to deal with desktop publishing. They teach you how to get control over the design process by verbalizing what you are trying to accomplish, and learning to trust your eyes. Also: by not being a wimp.
The Design Book is specifically about page layout (not graphic design in general), and mostly about laying out bodies of type. The most interesting thing about this book is that it (gently) rejects the idea that design is strictly a knack or an intuitive process, and emphasizes verbalizing what you are trying to accomplish. "Once you can name the problem, you can find the solution." (p. 10) "You must know what the rule is before you can break it." (p. 49) The book enunciates several principles of good design. Through many examples of bad design and better design the book shows you how to check whether the principles are being violated and how to correct the violations.
There's a very clear chapter on the categories of type (Oldstyle, Modern, etc.): how to recognize them and when to use them. Very Good Feature: each page names the typefaces used in the examples.
The only real weakness of the Design Book is the chapter on using color. It was clear enough but did not seem integrated into the rest of the book.
The Type Book is a much-expanded version of Williams's Mac is not a typewriter, The (2nd Edition) and The Pc is Not a Typewriter. Unlike the Design Book it is mostly concerned with type at the individual character level. It deals with topics such as correct punctuation, different kinds of dashes, and when to set punctuation in italic. It includes all kinds of fine-tuning of the appearance of the type, such as kerning, tracking, ligatures, swash characters, hanging punctuation, correcting widows, and balancing the appearance of ragged-right type. This book allows a more intuitive approach than the Design Book, urging you to "listen to your eyes" (a mixed metaphor, but effective).
I have only a couple of minor gripes with the Type Book. (1) I think it has more fine-tuning that a non-designer is really going to use (e.g., fancy ligatures and swashes). The number of tweaks is overwhelming and may make you feel guilty for not using all these features. (2) I love Helvetica, but Williams continually bad-mouths it, and this is wearying. Apart from these minor points it is an excellent book, clearly-written and full of solid and useful information.