My mom's Baptist church did an adult Sunday School study based on his book.
At this stage in time, I think it's best not to do group studies based on these people's books & materials...or at least limit it and keep more in
scripture which we know is safe for spiritual consumption.
I lead a book study at my Church. We have a mixture of people from those who are more mature to those who aren't. Also a very wide variety of backgrounds ranging from a few of us with charismatic to a couple with reformed and a few cradle episcopalians and a catholic.
We don't follow a curriculum or anything like that. The pastor selects a book that we think is good and we read it and discuss what we read.
The first book we did was knowing God by JI Packer. Very good, and very good for the people, especially those who are new to a more personal faith.
Now we are doing a book written by an evangelical about the value of liturgy. (beyond smells and bells: the wonder and power of Christian Liturgy) Its been good as well.
The value of doing books like these is in getting the perspectives and ideas of people who are worth listening to.
We could study knowing God in the bible, and we do through sermons and teachings, but the fact is that many people have a hard time understanding scripture and they either end up with a very basic, one dimensional view, or they begin to form wrong ideas because they are misunderstanding what they read.
The value of using a book by JI Packer is that Packer can put scripture in context (both of itself and its historical context as well). Packer has seen things in scripture that I have not, etc.
Through considering his ideas and his teaching, I get more understanding of scripture than I would if I simply read scripture alone.
I'm certainly not advocating that people should read books like this INSTEAD of scripture, but it is valuable to read them in addition to scripture simply for the sake of broadening our understanding.
Many people have the attitude that they don't need teachers, don't need other people because the Spirit will reveal to them personally and internally all that they need to know.
The problem with this is that it ignores a great deal of scripture, which teaches us, rather, that we as the Church need each other because The Spirit deliberately does not give all gifts to anyone person. He gives gifts to each so that we must grow together as a body.
As Paul teaches, can the eye say to the hand "I don't need you?". Yet that is the attitude many people have.
I am also very much a proponent of reading GOOD books, not just ANY books. But don't limit your definition of "good" to "that which agrees with me".
What I consider good are works that have stood the test of time and have a proven track record of orthodoxy.
This is true of some modern books like Knowing God etc, but I would ESPECIALLY recommend reading old books because there you not only get the perspective of different people, but also of a different time. This can help us over-come the foolish prejudice that most people have of regarding our time as the best and most advanced etc.
I find it ironic in a humorous way that so many Christians decry the wickedness of our time and talk about how these must be the end times because they are getting so bad.. but yet they turn around and practice a form of Christianity that basically was born in these times and regard older forms of Christianity as substandard. (this isn't meant to be a jab at your statements regarding this must be the end, it relates more to my personal experiences in my local area).