If I was asked to recommend one self help book to improve your whole life this would be it.
Its easy to read and the suggestions can be implemented if you take then step by step.
It deserves a couple of reads, one to get an overview and another slower read to implement change.
I have also listened to the audio book version (narrated by the author), which is also very good and more up to date.
Sadly, Stephen R Covey has passed away recently, but his legacy lives on through his books.