I was given this book over 15 years ago and it has served me very well, giving me great insights into other people's - as well as my own - behaviour . It is comprehensive and compassionate, with some personal examples given by the authors, which I always think is a nice touch. I have also read Don Richard Riso's version and consider Rohr & Ebert to be far superior. I found Riso's book patchy (some types described more comprehensively than others) and subjective (he seems to prefer some sub-types to others).
I just have two criticisms about this book: Firstly, Rohr and Ebert are, I think, Franciscan monks, so there is quite a lot of reference to religion, which I think is better left out of a general introduction to the Enneagram. However, you can ignore it, especially as most of it is contained in one chapter: Jesus and the Enneagram. My second reservation is that the English translation seems a tad clunky compared with the original German version. So if you speak German, go for that one.
If you want an easier and more light-hearted introduction to the Enneagram, start with The Enneagram Made Easy, but for serious Enneagram aficionados this book is highly recommended.