Having said that I found that the insights came thicker toward the beginning of the book than later on. I suppose that the interaction between the two was most intense when they were younger (the book progresses chronologically) and still developing than it was later on, when they had both developed their ideas more and were perhaps a little less jealous and competitive. The nature of their mutual influence simply changed over time, and the book reflects this. The last chapter, on Picasso's extended response to Matisse's death, is quite moving.
The book focuses very closely on the pictures in the show, and discusses them only as examples of this mutual influence. Sometimes this creates a slightly odd effect, as a painting is not analyzed in full, but only to the extent that it reveals the other's impact. One would sometimes like to know more about a painting than that, and to see it in a broader context; such is not the nature of the book, however.
These are very minor quibbles though, as the book is insightful and informative and contains some very good reproductions. It is also brief and briskly written, and I recommend it whether you've seen the show or not.