Excelsior

Dragonsinger, by Anne McCaffrey's Pern series. First library book I ever stole.
The Hobbit. Which lead to all the rest of the LotR books.
I Robot. by Asimov.
Starship Trooper. by Heinlein. It's probably is what got me some interest in politics. Culminating a few years ago with a run for city council. (I lost)
Harry Potter and the Philosopher's stone. by Rowling, and of course the rest of them. Got me writing fanfiction.
Striper Assassin, by Nyx Smith. Either that or "Never Deal with a Dragon". Shadowrun novels that got me reading fanfic in the first place.
Pride and Prejudice by Jane Austin. Actually the BBC miniseries from the 1990's roped me in to that one.
Honor of the Queen by David Weber. The second Honorverse book. Was a couple of years before I found the first lol Now read them all multiple times.
Magician by Raymond Feist. First book I ever got signed by the author.
Go Ask Alice. By Beatrice Sparks. Go read it!

Lucky 11, Mort by Sir Terry. Which of course leads to all the Discwork books

Stranger in a Strange Land - Heinlein
The title itself struck a chord in me and the book struck a deeper one.

Areopagitica - Milton
A bulletproof treatise on why no human has the right to force their will upon others. Like most philosophy, it's irrelevant in the face of reality but still a great read.

A Man For All Seasons - Bolt
I cannot help but be impressed by men who hold and adhere to principles in a world that rewards them so poorly.

The Communist Manifesto - Marx
I cannot help but be impressed that this man correctly predicted the future of capitalism (though not proletarians) 165 years ago and amazed that, despite this work, people today are still wondering what's wrong.

Don Quixote - Cervantes
It was written over 400 years ago but Cervantes had a sense of humor that would fit right in today. This book literally made me laugh out loud.

The Illearth War - Donaldson
First book I read in the Chronicles of Thomas Covenant series (and a good thing - I wouldn't recommend starting with the first). It was the first sword + sorcery novel I'd read that I felt borrowed nothing from Tolkien, and the series is so much more complicated and human than most. Sure, sometimes "I'm a Dragon Slayer, I must kill dragons" is entertaining but these provoke real thought and emotion.

Sappho and the Greek Lyric Poets - Barnstone
Beautiful no-nonsense works. Uninhibited and undecorated with gimmicks.

Tales of the Bounty Hunters - Various Authors
I'd actually never wondered what the untold stories of the Star Wars bounty hunters were before I saw this book. It was so well done though, that I became hooked and now own a collection of SW novels.

Mrs. Byrne's Dictionary of Unusual, Obscure, and Preposterous Words - Josefa Heifetz Byrne
I can understand that a lot of people might not care what callipygian or hippopotomonstrosesquipedalian mean - but, speaking as a nerd with Asperger's, this dictionary was a dream come true for me.

Mein Kampf - No book serves as a better example of men's legendary ability to justify anything to themselves and their need to do so, because even the worst monsters don't want to wake up, look in the mirror, and see a monster looking back.

Honorable Mention:

Grapes of Wrath - Steinbeck
Absolutely the most boring book I have ever attempted to read in my life. How this became a classic, I cannot imagine.

(English titles)

The Sorrows of Young Werther by Goethe
Candide by Voltaire
Story of the Eye by Georges Bataille
The Blind Owl by Sadegh Hedayat
Venus in Furs by Leopold von Sacher-Masoch
Maia by Richard Adams
Inferno by Dante Alighieri
The Metamorphosis by Franz Kafka
The Stranger by Albert Camus
Clarimonde/La Morte Amoureuse by Théophile Gautier

I feel like reading something now.

I have two seconded recommendations:

The Integral Trees by Larry Niven. In fact, a lot of the N-Space books (his fictional universe) are good, such as Ringworld.

Donaldson's Thomas Covenant books. I actually think Lord Foul's Bane, the first in the series, is the best one.

Here's a few of my favs:

Nonfiction:

Guns, Germs, and Steel: The Fates of Human Societies by Jared Diamond. This book is to history what Freakonomics is to economics. And honestly, it's a better book

"Surely You're Joking, Mr. Feynman!": Adventures of a Curious Character. This is an edited compilation of Richard Feynman's memoirs.

The equation that couldn't be solved : how mathematical genius discovered the language of symmetry by Mario Livio. Long title, awesome book.

The Tipping Point: How Little Things Can Make a Big Difference by Malcolm Gladwell. I found myself not agreeing with his conclusions, but he makes his case so eloquently that this book was hard for me to put down.

Fiction:

The Terror by Dan Simmons. Fictional account of the loss of the HMS Erebus and HMS Terror, which went missing while trying to find the Northwest Passage in 1845 or so. This is a "historical horror" novel (my invented term!). There's a creature, and isolation, and lots of bad food.

Hyperion by Dan Simmons. My favorite book by Simmons. He's such a good author, his work merits two entries on my list. This one is a soft scifi space opera that is organized like Chaucer's Canterbury Tales. It tells the story of pilgrims on a space ship bound for the planet Hyperion. Like most of Simmons' work, it's a mashup of horror, scifi, and intertextuality.

Watership Down by Richard Adams. I adore this book. I've reread it more times in my life than any other book. It's just ... a classic.

Preludes and Nocturnes by Neil Gaiman. Okay, this is a graphic novel. And yes, I'm a 40-something dude who reads comic books. But seriously, read Gaiman's Sandman graphic novels, of which this is the first, and tell me that this isn't a serious art form.

The Anubis Gates by Tim Powers. A body-hopping werewolf, the Egyptian underworld, time travel, and a bit of romance.

Jumper by Steven Gould. Ignore the excrementitious film version of this book. It's a great read.

Cryptonomicon by Neal Stephenson. A fantastic, fictionalized ride through history following the role that cryptography plays in deciding the outcome of wars and individual human lives.

Artemis Fowl by Eoin Colfer. Tired of that goody twoshoes Harry Potter? Ever wonder what Stewie from Family Guy would be like if he were rich and kidnapped elves? Well, read this book.

The Call of Cthulhu by H.P. Lovecraft. I'm a Lovecraft-o-phile. Some would say At The Mountains of Madness is his best work. They would be wrong. This short story, which is arguably the definitive work of "wierd fiction", has been eating the souls nerds like myself for decades. Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn!

Dune - Frank Herbert
Inferno - Dante Alighieri
Othello - William Shakespeare
Anarchism: a collection of revolutionary writings - Peter Kropotkin
The Complete Joy of Homebrewing - Charlie Papazian

Thought I would list a few I loved that I didn't see listed.

Magician, by Raymond E. Feist. First book I ever read (later split into 2 books, Magician Apprentice and Magician Master) Totally got me hooked to reading

Dungeons and Dragons books by Gary Gygax, got me started on my long love for RPG.

The Belgariad by David Eddings, for countless rereads. The characters are like old friends.

A Spell For Chameleon by Piers Anthony, This and all the other xanth series were clever and funny.

The Integral Trees by Larry Niven, and the sequel The Smoke Ring. A great concept for human colonization.

The Hobbit by J.R.R. Tolkien, for setting my feet on the path for the whole epic of LOTR and the Silmarillion.

Eye of the World by Robert Jordan, and the rest of the Wheel of Time series. Years of waiting for the next book, and the next book, kept me thirsty for more.

The Celestine Prophecy by James Redfield, for changing perspective on our past, and perhaps our future.

Clan of the Cave Bear by Jean M. Auel, for reminding me that our past is so much more than the written history.

Fundamentals of HVACR by Carter Stanfield and David Skaves, my current reading project. To get out of labor jobs and into a skilled trade.

Honorable Mention, Daughter of the Empire by Raymond E. Feist, a spin off series from the Magician books, great story.

Magician - Raymond Feist was mentioned - all first 21 books rock - he started losing it in Wrath of a Mad God though.
A dragon in the garden - Reginald Maddock - a small novel with hidden meanings that's been a favourite for almost 20 years.
Dead Witch Walking - Kim Harrison - Again the whole series is a good read.
Pacific Vortex - Clive Cussler - again anything by cussler that features the hero "Dirk Pitt" is worth reading. His Kurt Austin books are a shameless attempt to keep in the best sellers list.
The Colour of Magic - Terry Pratchett - already mentioned but all his discworld novels are quite witty - and considering his Alziemers a major achievement.
Across the Nightingale Floor - Lian Hearn - The Trilogy beginning of Tales of the Otori.
Harry Potter Series - J.K. Rowling - Yet another mentioned but started reading this to my daughter and became hooked myself.
Dragons of an Autumn Twilight - Margaret Weis and Tracy Hickman - The first of the Dragonlance novels.
Druss The Legend - David Gemmell - Can you guess where I got my characters name from? All his books are fantastic.
and finally...
Dragon Prince - Melanie Rawn. A six part series that was both complex in characters but simple in the Idea - her second series began on a completely different tangent and had promise but illness struck her about 1/3 the way through the final book in the series and we are hoping that she recovers enough to finish it - although after 9 years, it seems unlikely