This isn't so much a book really but is a collection of material from different places by Katherine Chernick Fauvre on the Enneagram Instinctual Subtypes (so,sp,sx). She did a study and published these results in 1995 and it is a compilation of what others have said.

It starts out with some high level perspectives on the topic of subtypes and then is followed by what look like several PowerPoint slides. From there, it goes into the "profiles." There is a page or so of bullet points for each Enneatype + instinctual subtype combination. So that is 27 pages or so of bullet points. In addition to that, she includes extensive references for each one. So that is about another 27 pages of references where you can find more information.

What I like about this is that it provides more information in one place about each of the instinctual subtypes and what it looks like for each of the enneagram types than any place I know. What I didn't like is that the material, which is mostly bullet points, which was pulled from many different places, naturally seemed somewhat disconnected. I got a lot out of it anyway, regardless as the the form that it was written in.