My gateway book was The Sword of Shannara by Terry Brooks. My brothers gave me the trilogy (Sword of Shannara, Elfstones of Shannara and Wishsong of Shannara) for Christmas one year because they liked fantasy and they wanted to read the books.
I was probably around 9 or 10 at the time and not at all interested in fantasy books. I'd read The Hobbit and liked it, but not enough to look for other books of the same style. And I remember reading a book when I was a fair bit younger than this called The Seventh Princess by Nick Sullivan. I enjoyed it (and hunted it down as an adult) but again, it didn't inspire me to read other fantasy books. I was also an indifferent reader at the time, preferring Christopher Pike to anything else. So the Brooks trilogy sat on my shelf.
I remember picking the first one up once and reading the back. The idea of a man without training in magic or swordmanship skills going against the most powerful evil in the land (and presumably winning) struck me as unrealistic. I tried a few pages and found it boring so I put it back down.
Finally, when I was 12 going on 13, I hit a rough patch in my life and needed some escapism. Maybe it was simply a matter of the right book at the right time, but I started The Sword of Shannara and pushed on past the still boring opening. Somehow, this time, about 20 pages in I got hooked. I couldn't put the book down. I read the 730 page novel (probably the largest I'd tried up to that point) in record time (I later clocked myself at 2 1/2 days - but that was when my reading skills were better).
I loved the book. The protagonist was real to me. His struggle to learn magic in the face of feeling like a useless appendage to the more experienced members of the group resonated with me. I liked the flowing descriptions. I liked Menion Leah - the comic relief who's there to prove that he can be more - and who, when faced with murdering someone in cold blood to help his friends, can't do it. I liked the philosophy and the recognition that everyone faces challenges. And that with help and perseverance those challenges can be overcome.
The book was full of difficult choices for the characters. There was action but there was also time for the characters to consider their actions and the consequences of those actions - both immediate and long term.
In the end, it was about a man who has to become a hero, despite a lack of ability and courage.
All of a sudden I was a voracious reader. I ripped through the rest of the trilogy and then moved onto my brothers' collections and when I'd exhausted those, the public library. Tolkein, Feist, Salvatore, Eddings, Weis & Hickman, McCaffrey, Greenwood, no one was safe. I spent the entire summer reading, and when I started highschool I branched out and found more and more authors. I've never looked back.
I've read The Sword of Shannara so often I can't read it anymore (as I know what's going to happen at every point). But I still have that copy (autographed) and it still inspires me. And someday I'll read it again and feel the magic I felt that first time.
In the meantime, it has opened the door for hundreds of other fantastic authors and stories. Oh, and I never found the opening boring again, even though I've read the book numerous times since.
So, what was your gateway genre novel?