For me it would have to be this:
If you read into deeply it really gives you insight into the human condition - not only then but even today.
Maybe Scott’s Standard Postage Stamp Catalog, when I was a stamp collecting child. That enthusiasm taught me most of what I know about history, geography, and quite a few biographies, and even some natural science and economics…
As for conventional reading books, maybe “Let Us Now Praise Famous Men” by Jame Agee.
Interesting choice (not being critical at all - meant as a compliment believe me - I certainly never would have thought of something like that).
“The Power of Now”.
Was an easy choice.
Titan by John Varley.
After I read it, I started writing seriously. I still can’t explain why it had that effect on me.
(Though it occurs to me that the lead character is something of a template for several of mine.)
One Flew Over The Cokoo’s Nest. Absotively.
And, in before the Fuming-Anti-Woo-Brigade shows up:
That’s a good one, too.
I’ve mentioned it many times before: Learned Optimism.
Use it in some way almost every day of my life. I had read and even worked through other Cognitive Behavioral Therapy books before, but nothing ever took. Then I read Learned Optimism, and even though all of the corporate speak wasn’t necessarily relevant for me, somehow the message got through. The subtitle is “How to Change Your Mind and Your Life”, and it did that for me.
“Atlas Shrugged.”
I know this violates Panache’s Law, never ever mentioning Ayn Rand on these boards, except critically. But she - both personally and through her books - was a tremendous influence on my younger self.
And I agree with jtur88. So many things I know, I learned through collecting stamps.
Not going to name it 'cause it was not a very good book. Written by a friend of my parents. He had been trying and submitting and getting rejections and finally he sold one. Of course we got a copy.
He was a twit. But immediately after selling this book, he bought a house. A really nice house. I think he may have also got a movie deal, but as far as I can tell no movie was ever made of this book, or the two sequels (I am thinking he got a three-book deal).
At the time I was a kid who idolized writers and thought they were gods. This experience of my family’s friend taught me three things.
- Writers are not gods and can be very ordinary, and even twits.
- It doesn’t have to be a great book, you can sell it anyway.
- There’s money in it.
Armed with this information I decided to become a writer. My basic idea here was if this twit can do it, I can do it. I was probably going that way anyway but this made it all look a lot more possible. And I did sell some books.
But no, I didn’t get a house in La Jolla out of them.
Yet.
Foucault’s Pendulum, by Umberto Eco. Not the easiest thing to read, just from the sheer density of allusions and references; but his explanation of the human origin of archetypes, the psychology of conspiracy theorists, and the importance of skeptical thinking very much shaped my world view.
My mom got that book for me when I was thirteen! It very much influenced my politics, in later years - made me distrusts conservatism and worship of the "good old days’.
Catch-22. My first of many readings was 40+ years ago.
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“Arsenal: Understanding Weapons in the Nuclear Age,” Kosta Tsipis, 1983. Written when full scale nuclear war was a possibility, this book turned me away from war completely. I’m pretty much a rabid pacifist now.
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“Cadillac Desert,” Marc Reisner, 1986. An examination of Western water issues at the end of the American dam building era. Gave me a new perspective on water rights, access and use.
(First editions of both are on my bookshelf.)
All Quiet On The Western Front, read in my teens, helped shape my views on the stupidity and futility of war.
*another work by the same author, Erich Remarque (Arch of Triumph) was also influential. Unfortunately there were no local cafés at which I could hang out while bemoaning the vagaries of life and drinking calvados. 
Mine was “Think and Grow Rich” by Napoleon Hill. Corny, I know, and probably showing its age. I’m sure it’s still available. I was probably 13 years old, and came away impressed by his encouragement of how to put agressive plans together and then successfully execute them.
This book was probably more of an eye-opener to related subjects that became my career.
Formal Languages and Their Relation to Automata by Hopcroft and Ullman.
I had thought I wanted to study compilers in grad school. Once I discovered this, I knew it was the formal languages side of things was what interested me. Which lead to …
In short, a lot of stuff changed for me from that.
I’m an avid reader of mostly epic fantasy but because you asked for the “single most influential book” I’d have to say Allen Carr’s Easy Way To Stop Smoking. I read it, then stopped smoking so I guess you could say it greatly changed my life - that’s pretty influential!
A few, none of which have been a huge influence, but still:
The March of Folly by Barbara Tuchman had a passage in it implying that the perceived systemic corruption in Renaissance Italy couldn’t have been all-pervasive, or there wouldn’t have been anyone condemning it. That can be applied to a lot of situations, including Purple America, since districts that vote 90% in one direction rarely have anyone competent contesting them, and the opposite of the maxim is also useful to prevent looking rosy-eyed at the past and overlooking the probable rampant sexism and ethnism*: it was probably much more common than was written about but no one saw anything wrong with it.
*I use this term because “racism” today refers to the 19th century concept of race, and few people historically even met someone of a different race, by that definition. But I’d have to guess that prejudice against other ethnicities was probably rifer back then, since people were less cosmopolitan.
Does the Straight Dope column count? Cecil’s column about furnaces has a very profound quote: Little things give us little problems. Big things give us big problems. That maxim, too, is useful all over the place when someone tries to stretch an analogy too far in support of or against their pet policy.
I haven’t made money writing, but I recently came to this same conclusion when I tried to read Infinite Jest. Up until that book, every popular book I’ve tried to read had been either brilliant or pure dreck. The pure dreck is more easily overlooked as perhaps not written for me, but a not-bad but ultimately not-good book that nonetheless is popular does more to remove the illusion that successful writers possess god-like talents.
‘The Jungle’ by Upton Sinclair , I stopped eating meat after reading it. When I went to college a teacher got talking about this book and said it was an influential book and I agreed with him .
On a personal level reading ‘Excession’ by Iain M Banks changed my life, before the advent of Amazon I liked to browse second-hand book stores, I picked this book up because of its cover and I liked the summary but it was just a little more expensive than I could justify, I almost set it down but changed my mind at the last moment and bought it.
It was the introduction to the whole ‘Culture’ series of novels and literally changed the way I view the world.
I would have been about 16 at the time, it probably wouldn’t have had such a profound impact on me if I had read it later in life.
*The Timeless Way of Building *by Christopher Alexander. If there’s such a thing as a religious text for me, that’s it (and it’s even written in a chapter and verse kind of way, that makes me want to say “amen” at the end of every section). It’s exhortational, reassuring, and instructive. It describes life the way it ought to be lived.
I’m happy to say that about a decade after my first reading, my life and home are following a lot of its statutes.