If you were allowed only one book on typography, it should be this one. Bringhurst is a poet. He loves language, written language, and all its parts. That love comes through in the text and the visual presentation of every page.
Bringhurst advocates a subdued typographic style. This makes good sense in the vast majority of cases, since typography is the servant of the text that it carries. Like any good servant, it should be unobtrusive, well dressed, and competent to handle every task it is given, quietly and promptly. Bringhurst demonstrates nearly everything he says, starting first with this book itself.
The book is a beautiful artifact, with an elegant and informative page layout. Body text, side- and foot-notes, references, running titles, and more - they all fit together well on the page. Each kind of information is set off only slightly, but clearly and predictably. The content is well organized: prose in the early chapters, reference material in the later chapters and appendices, and all the intermediates in the middle of the book. Diagrams and tables are minimalist and communicative.
The text spans centuries, from ancient Egyptian page layouts to the rationale behind Unicode. Bringhurst is passionate about typography's history, and insists that it inform every modern decision about print and printing. He embraces the new just as much, and is careful to note the strengths and weaknesses of each typographic technology.
Bringhurst discusses far too many topics to touch on here. In every case, though, he brings his poet's sense to all of the writing, using witty, descriptive language for even the most mundane of technical issues. The one weakness I saw was in the geometry of page layouts. I like his mathematical rigor and esthetic practicality. Still, I think that the number of different constructions was more a tribute to what can be done than to what serves a real need.
This is the best, most complete text I know on book design. As Bringhurst points out, there are lots of other uses for type than books, but he chose books as his subject - I have no problem with that limitation. The only problem I saw, and not really a problem with the book itself, is its subtlety. The nuances (well, most of the nuances) he discusses are important. Beginners, however, may not see the significance of small matters. Once a reader's eye it tuned to the fine detail, however, this book is the most helpful I know.