First and foremost, this book has a definite audience: people who need to produce graphs for somewhat sophisticated audiences. This is not a book about producing graphs for mass marketing or other flashy arenas. While this point is implicit throughout the book, it is not often stated explicitly.
The biggest strength of this book, and what makes it worth the purchase, is Cleveland's discussion about the relationship between graphing and visual processing. We've all seen a thousand pie charts, for example, but it turns out that people are not good at visually processing pie charts. The way we process visually has implications for everything from line graph construction to color choices to deciding how to code data on XY scatter plots. Although this information does exist in other places, Cleveland brings it together concisely here. Some of the discussion can get a bit technical, however, so be warned.
This is a great first book to read to learn more about how to construct graphs, and it has enough references to point you to other sources if you feel you need more. I myself have purchased several other books about the visual representation of data (including Cleveland's other book "The Elements of Graphing Data"), but this is where I started, and the information in this book has enriched my understanding of those other books immeasurably.