I've read Personality Type, Gifts Differing and 8 Keys to Self-Leadership by Dario Nardi.
I voted for Thomson's book because it extensively defines the functions and explains how they effect behavior. Personally, I thought she did a good job of not generalizing behavior that is really just specific to the individual. Instead, she provided reasoning about how those common behaviors come about as a result of the functions interacting with each other.
Honestly, I had to read the INFP profile a couple of times over before it truly began to resonate with me. Once I did, however, it was surprisingly insightful in its explanation the effects of the tertiary and inferior functions. That being said, INFPs who are Nines on the enneagram may have an easier time relating directly to the descriptions, I think. One problem, I had, though, was the she wrote extensively about the importance of developing the auxiliary function, but her advice for doing so is very opened. But really, how could I expect a mathematical formula?
Nardi's book focuses exclusively on defining the functions using more technical language as well as providing many exercises for developing each one. So, in this respect, it's a helpful guide because the material is so concrete. Unfortunately, I became skeptical of his information when he started to take the "mystical" approach to introverted Intuition. Unfortunately, much of his writing directly opposes that of Thomson's. For example, he says that extraverted Feeling "requires us to be personal, compassionate and without hard boundaries" (123). On the other hand, she writes: "Ultimately, a exclusive reliance on Extraverted Feeling leads to anything but a reliance on emotion. Extraverted Feeling types ignore their immediate impressions and focus only on social obligations" (321).
Additionally, he recommends focusing development on the auxiliary, tertiary and inferior functions, while Thomson says to focus only on the auxiliary.