Hmmm, I would say that "A Wrinkle in Time" would be the first book I read that I became engrossed in. I was also in 1st grade, so I missed quite a bit of what was going on.
Grapes of Wrath would be the first highly influential book I read, funny how I thought I knew about life in 9th grade.
The most influential book in my day to day life outside the big one is "The Servant" by James Hunter.
I have enjoyed most of the Ender Universe series and actually agree with Angie that I liked the Speaker arc much more, although I also think the Shadow arc would make for much better movies. I loved the Speaker arc pretty much all the way.
Also enjoyed the Circle series by Dekker, the Lost Books series to a much lesser extent.
I didn't mind the Scarlet Letter, although I will never choose to read it again.