I agree with F@ about the distinction between influential and inspirational. I also can't limit it to one book, as different parts of my life have drawn inspiration for different books. A lot of my list mirrors some of the things others have listed. Most have some influence on my beliefs as an anarchist in some way (since I assume that is why it is being asked on @101). I am trying to list them somewhat chronologically.
If I am really digging deep into what got me where I am I would also have to agree that Ishmael by Daniel Quinn definitely was the first book that critiqued civilization in a way that put to words a feeling I had held for most of my life. In a way it was confirmation bias that made it so influential, but it really was this moment where I was like, "oh, me and my one friend who I talk about this stuff with aren't alone!" This one predates my using the a-word to describe myself. It is also the book that ultimately led me to make the dietary decisions I've lived with for more than half my life.
Get In the Van by Henry Rollins (yes, that Henry Rollins) had a major impact on me in my late teens. It came out at a time when I was in a punk band with my closest friends, and his diary of touring for several years with Black Flag was absolutely a catalyst for us to place the band before jobs and other things, even though he comes off as an asshole a lot of the time in the book. That band didn't work out so well, but those of us who were reading it at the same time carry some of the stuff we took from it to this day, for better or worse.
Shibboleth by Penny Rimbaud (of Crass). As a young anarcho-punk, Crass had a huge influence on me, but I always grappled with the pacifism the band espoused. In the book, Rimbaud expressed some of his own tensions, and his rejection of the term. That was a really big shift for me.
Anarchism and Other Essays by Emma was the first explicitly anarchist book I ever read. Bought it at a bookstore owned by folks who had once been part of the Left Bank Books Collective. It was not an anarchist store, but they had lots of anarchist books, and I knew a little about Emma (enough to get that book), but buying that book was kind of the moment I crossed a threshold from being part of a small group of anarcho-punk friends into... whatever it is that I am now.
A People's History by Zinn is a sort of obvious one. For me it shifted my understanding of the historic lineage (I sort of cringe at using that word) of resistance in the US.
Mutual Aid by Kropotkin was another major influence. As I've already sort of mentioned, I was skeptical of civilization before I was an anarchist, and so Kropotkin might seem like an odd entry to this list, but despite his enthusiasm for progress as a liberating influence on human society (and his enthusiasm for society), I gleaned a lot from this book about, well, mutual aid.
It ain't a book, but AJODA goes on this list. I'm not sure where to put it exactly, chronologically. I first encountered it around the time I bought the Goldman book (maybe before) I was fishing for anarchist writing, and picked up copies of Love and Rage, Fifth Estate, Alternative Press Review, and AJODA. It was in these pages I first encountered the writing of Jason McQuinn, Paul Z Simons, Bob Black, Lawrence Jarach, dot matrix, Aragorn!, and so many more. At first I didn't know what to make of it all. What was this post-left thing? But over time, I found that the problems I had with my lefty anarchist friend's politics were a lot of the same things that the editors and contributors to AJODA were exploring, just they had a few more years of having been delving those depths.
Elements of Refusal is a book I have come back to repeatedly. It is Zerzan at his best, in my opinion. Moreso than any of his other work this is the one I tend to pick up when I am thinking about anti-civ stuff and have that feeling that the Zerz at some point addressed it. In fact, I have two (maybe three?) copies so I can loan it out (well also, because my 90 something year old grandpa found a copy somewhere and was like, "my grandson would like this. No idea if he read any of it or just thought it looked like something I'd read, he also gave my niece a ski mask at the same time, honestly, I don't know what was going on there, but it was so perfect that this old man who grew up in rural Central Washington at a time when agriculture was still horses and child labor found a copy of one of the most influential books on my thinking and gave it to me that I decided having two copies was a good plan, plus my original was starting to fall apart from rereading).
Homage to Catalonia was a book that I read well after I started to call myself an anarchist, but which I still found both tragic and inspiring. I don't know that I can say why, but it did.
Against His-Story, Against Leviathan by Fredy Perlman was another book that shifted my thinking in a very subtle way that I can't quite explain. I'd already read Zerzan, I was most decidedly anti-civ in my outlook, but there was just something about this that.. I don't know.
Those who are on here a bit already knew this was coming... Prison Memoirs of an Anarchist by Alexander Berkman. It is my favorite book. I've gushed about it elsewhere, so won't repeat myself, but I will leave this (about his meeting EG after his release from prison): "We walk in silence. The Girl a bouquet into my hand. My heart is full, but I can not talk. I hold the flowers to my face, and mechanically bite the petals."
Where We Stand: Class Matters by bell hooks. This bell hooks title makes the list because it was the first one I read (for a college class that didn't suck). While it is primarily her attempt to look at issues around class in the US, it is (of course!) infused with her feminism. She's a liberal/progressive, but her work resonates every time I come back to it.
The Politics of Individualism by L Susan Brown. Brown calls herself an anarcho-communist, but Murray Bookchin hated her, so she must be doing something right. I read this in a short-lived anarchist reading group I was in, and appreciated her outline of a difference between liberal and existential individualism.
State Structure and Genocide by Andrew Kolin is a short, perhaps overly academic book which cohesively lays out an argument that genocide is the logical conclusion of the state apparatus.
Letters of Insurgents - I read this about a year after some folks did the Insurgent Summer. It was challenging, inspiring, and heartbreaking all at once. It isn't cheating to list two Perlman books is it?
The DaVinci Code by Dan Brown (I kid!)
Blessed is the Flame by Serafinski is the last book I'll list. I was already familiar with, and influenced by, nihilist strands of thinking, but this book did (for me) a much better job of really bringing up questions that I still grapple with about what it means to struggle against forces that almost certainly will be one's demise. The sand book might be more timely in addressing ecological catastrophe, but framing hopelessness in the lens of concentration camp resistance during the Holocaust was what really resonated more with me. (I appreciated Desert more upon rereading it after Blessed is the Flame.
Gah. I love books.