You should have asked "other than the Bible" because so many will go for that low hanging fruit. We need an "in addition to" book. And don't get me wrong, mine is on my nightstand, but it's kind of an obvious answer (for not all, but many).

The BIBLE!

There is a reason it's been a Best Seller for years. If people would pick it up they wouldn't need the self-help gurus.

No book has changed my life. 

I have applied the wisdom I acquired from reading many books and I changed my life ... not the book. 

One good book led to another and then became so many. I learned bits & pieces from them all too. Interesting to note that at some point: I wrote six books of my own that have influenced people's lives & still do. Life is good

The Bible.  No other book guides my walk with God and keeps me on course to do the things He's called me to do.

#1; The Bible

#2: The One Minute Manager

Business-years ago at age 22 , I read “See You at the Top” by Zig Ziglar

Just one. Hard
"The Greatest Miracle In The World" Og Mandino

The Holy Bible

I used to read a LOT and I am not interested in fiction books, so I would not be able to pin only one single book, since there are hundreds I love. 

Aside from the Holy Bible?

        Books that have changed my life (I read both of these when I was a teenager):

    None Dare Call it Conspiracy by Gary Allen

    How to Win Friends and Influence People by Carnegie

My library is vast and there have been so many that have contributed in so many ways.  Seems like I am gaining a new gem of knowledge every time I pick one up.

Haven't found that book yet.

The Dynamic Laws of Prosperity - Catherine Ponder

Magnetic Marketing - Dan Kennedy

The New Physco-Cybernetics - Maxwell Maltz

Rich Dad, Poor Dad (Robert Kiyosaki) - absolute game changer not only for our own personal lives, but many real estate investors have read this book (so professionally, it's a good idea to be versed in the teachings for conversational & strategic purposes)

"Swim With the Sharks"  Harvey Mackay

The 4:2 Sales Formula by Jeff Shore.

Codependent No More. Getting yourself straight does wonders for every aspect of your life- personally and professionally.

I started working before "women's lib" reared it's head, and had to fend for myself in corporations with pretty much all men in charge.  So, ludicrous as it might seem now with the "Me Too" movement, I read the Helen Gurley Brown books on how to survive in my personal and business world.  My path in those days was international advertising and public relations in the tourism industry. 

Any Robert Schuller books or Hill books

Siddhartha a Novel by Hermann Hess. A story about going from perfect to searching for a better life to realizing how good it was in the beginning and trying to get back to it.