Mr. Tognazzini is clearly a talented designer, but as a writer, this collection of newsletters, aphorisms, and anecdotes is kind of all over the place. A lot of it is very specific to the Mac interface at the time of printing (1992), most of which I had never heard of, so a lot of the examples fell flat when I understood them at all. Most of them assume a familiarity that I simply do not have.
Other parts of the book are excellent - chapter 17, on conceptual models, could probably be the basis of a pretty spectacular interface design book on its own. The guidelines, peppered throughout the text, suffer the same problem as the letters - sometimes they are fantastic and useful to any designer, but other times they are overly specific to the exact circumstances of the writer.
Ultimately, it seems like the field of interface design is still in its infancy, and books like this show the process by which anecdotes become generalized into theories.