I dont really have a FAVORITE book. Although I spend a fair amount of time reading dog training books. One of my favorites has to be "What Color is Your Dog?" by Joel Silverman
But one book I read in High School, at the time gave me a fresh insight into living my life. While I do NOT profess to be a "Bible Thumper", a "Model Christian" or for that matter even a "faithful church goer" ... I did find a lot of what it said to be helpful in living my life. That book is called "The Screwtape Letters" by CS Lewis.
The Screwtape Letters is a Christian apologetics novel written in epistolary style by C. S. Lewis, published in 1942. The book takes the form of a series of letters from a senior demon, Screwtape, to his nephew, a junior tempter named Wormwood. The letters advise him on methods of securing the damnation of a British man, known only as "the Patient."
Screwtape (along with his trusted scribe Toadpipe) holds an administrative post in the bureaucracy of Hell. He acts more as a mentor than a supervisor to Wormwood. In the body of the thirty-one letters which make up the book, Screwtape gives Wormwood detailed advice on various methods of undermining faith and promoting sin in his Patient, interspersed with observations on human nature and Christian doctrine.
Lewis provides a series of lessons in the importance of taking a deliberate role in living out faith by portraying a typical human life, with all its temptations and failings, as seen from the demon/devil's viewpoint. Wormwood and Screwtape live in a peculiarly morally reversed world, where individual benefit and greed are seen as the greatest good, and neither demon is capable of comprehending or acknowledging true human virtue when he sees it.
The book is available online. Just search "Screwtape Letters"