The book, and Tufte, focuses on effectively displaying quantitative information. He takes a relatively hard line about it and I have found it one of the most useful texts for helping me take "some truth the system knows" to "some truth the user knows".

Of course you have to take it with a few pinches of salt.

Firstly it is largely his opinion. Studies into this area are rather good on the general points however the fine detail is incredibly hard to study accurately.

Secondly you may not actually want to make a UI that conveys the "truth it knows" above all else. Often you want to convince the user that the UI is good at conveying information (which is not the same thing) or that it is very easy to use (which is again, not the same thing).

I heartily recommend this book because it teaches one very hard thing very well. You just have to understand that you do not always want to do this thing (yet you now have a way to start to understand the trade-offs you are making).