The "Gateway Drug" of Speculative Fiction

Instead of posting a review today (I’m having another bad energy day so I just don’t have it in me) I’m going to ask a question of my dear readers about what YOUR gateway drugs of speculative fiction are. Continue reading to see what I’m talking about. 
I was online talking to a fellow speculative fiction reader (@pbdp on twitter) last night when we got into a discussion about the books we read that got us into speculative fiction. I mentioned to him that my first book that got me into SF was The Mists of Avalon by Marion Zimmer Bradley. Because it was my first book of this nature, it will always be considered fantastic to me even though it’s probably really not that good. It’s more of a sentimental thing than a quality thing. 
He replied by saying that he thinks everyone has a book like that; a book that’s not that great but holds some sentimental place for us as it got us into SF. Generally, I think we tend to think of these books as being better than they actually are. His was The Sword of Shannara series by Terry Brooks, which many complain about but he will always consider awesome because it was his first real SF series and it got him into the genre. 
So, you have our answers. My gateway drug for SF was The Mists of Avalon by Marion Zimmer Bradley and @pbdp’s was The Sword of Shannara series. What’s yours?