At first I was tempted to be snarky (I know, what's new with that?) and go with the phone book. No one really uses them anymore but I definitely got quite a bit of use out of them back in the day. Before the internets, they had the best collection of information. Everything from maps to seating charts for theaters to school district information and more. You could use them to show feats of strength, get a higher seat for shorties, hold down papers needing glue to dry and provide hours of entertainment for crank calls.
But you didn't really read the phone book, now did you?
I'm not one for self help or business books so none of that works for me. I'm not religious and while I've found some useful things to ponder in various ethical writings, I'm thinking the questioner is referring to more practical matters.
I'm a teacher first and foremost and while Stephen Krashen's The Power of Reading: Insights from the Research
It's a slim book packed with great anecdotes about what works in reading, backed up by much research. If I could make sure all teachers and librarians and administrators read one book, it would be this one. (For parents, I'd probably go with The Read-Aloud Handbook, which also refers to Krashen).
This book made me a better teacher. What's more useful than that?