I don't know that the forty days is a hard and fast rule...I think  that the
author is just trying to get people to think of or about their relationship
with God every day...maybe he thinks if one did this for forty days it would
create a habit.
It is true the book is very basic, God loves us, we were created for a
purpose, you are not an  accident God has a plan for your life while I know
all of this is true, it is still good to remember that the hard times in
life aren't just happenings, there is a p plan to the seeming madness of
living, 
Rhonda