M.S.
My 9 year old loves the Diary of A Wimpy Kid series and anything having to do with history. My 11 year old boy read two books out of the Fablehaven series and LOVED them. He has started Harry Potter, but only got to book 4 because it got a little too intense for him. However, he really enjoyed Fablehaven.
For you, if you haven't read Harry Potter, they have to be my all-time faves. I also like the Sophie Kensella books. I really liked The Lovely Bones (read it a couple times). I have horses, so I really, really loved Chosen By A Horse. Even if you aren't a horse-lover, I think you'd enjoy it. In my family, my mom, two brothers and sister had all read Devil in the White City and loved it. It didn't really sound like my kind of book, but they talked so highly of it, I decided to try it. Wow. I loved it. It's about the building of the World's Fair in Chicago. It also chronicles the life of a serial killer at the same time. It was awesome!! I had only wished I had read it before we vacationed in Chicago just weeks before. Oh, and since you have girls, I have to strongly recommend Odd Girl Out by Rachel Simmons. It talks about girl bullying and how it effects their self-esteem. I recommend it to any mom with a girl!
Good luck and happy reading!