I would have to say Bloodlines for sure. I know it comes out this month, but I'm dying to read it now. If I had to choose one that would be awhile though probably Wings of the wicked (the second book in the angelfire series) by Courtney Allison Moulton. I'm reading Angelfire now, and enjoying it very much.
I can't think of one book that has changed my life or influenced in any way. I do know that some of the first books I picked up, and decided I would try and read were Mary Higgins Clark. I read Daddy's little girl, Where are the children, and a few others that she wrote. Then my friend told me I should read The Deep End of the Ocean by Jacquelyn (I may have spelt her first name wrong) Mitchard. It's about a little boy who was kidnapped, and returned to his family after 9 years I think. It was really heartfelt, and to see the struggles that the family went through. It was definitely a good book (very thick too).It was also made into a movie.
What got me into YA though is Twilight. That's one thing I can't deny is Twilight opened my eyes to others. I read Harry Potter before, but it's not what got me into it. Although I'm not totally obsessed with Twilight, and there are better books. I don't hate it.