I'd actually ordered this book on Amazon before I read a reference to Edward R. Tufte in Slide:ology, and so I was immensely happy when I finally got my copy in the mail. Envisioning Information is packed with rich examples of presenting complex, layered data in ways that are usable and even visually pleasing. I recommend this book to any one interested in usability and interface design.
Be warned though, that this book is dense with ideas, explanations, and images. It reads more like my college biology text book than a normal design book, and the vocabulary is often times as complex. Reading this book can be daunting, but so far, the value I've gotten back out of it has been immense.
I think my only complaint about this book is that it is beginning to become dated. Written in 1990, Tufte spends most of his time talking about designs created for printed media, and the examples of computer displays seem very primitive by today's standards. Luckily, media is no impediment to the concepts in Envisioning Information; "flatland" and "chartjunk" are the same if on a poster or on the web.