I don't think there is a great deal of redundancy in the books.
ISoCF deals almost exclusively with refuting JW doctrines from a biblical standpoint, by showing how they misinterpreted, or otherwise have taken scriptures out of context to fulfill their own purposes. Franz includes very interesting antecdotes, including some Governing Body conversations, to buttress his points. I personally enjoyed the chapters on blood, and how they misinterpret the "preaching mandate", the most.
CoC also deals with some doctrinal issues, but not so much, and they are seperate issues from what was talked about in ISoCF. CoC basically covers the reasons for Ray Franz eventually leaving the JW's, including some very glarinly unjust policies (the Mexico/ Malawi situation really upset me when I read it). The final chapters recounts Franz's explusion from the religion in copius detail; I practically could not put it down when I read that part initially, and I've re-read that part of the book 2 or 3 times subsequently. He covers the 1914 doctrine just superfically enough for you to understand its gravity, but Carl Olof Jonsson's book is a better resource on this particular issue (and infact is recommended by Franz).
Even though CoC is the older book, I read ISoCF first because when I ordered the two books together, it only took 2 days for the bookstore to get ISoCF ( another local outlet had one), but CoC took 7 weeks to back order from the publisher, as they were totally out of stock. I didn't loose anything by reading them in that order, because as I said earlier, they really deal with seperate issues altogether.