I loved the Betsy-Tacy books (by Lovelace), and they will grow with the kids (eg., the stories start at age 5, and end when Betsy is married) - i read them all through jr high and highschool (and just re-read them).
I also loved the Anne of Green Gables books, and, again, i think they can grow with the kids. I think Anne is 7 or 8 in the first book, and the series follows her until she is married with kids. Again, frequently re-read in my house.
Jean Little wrote a bunch of books i liked (From Anna, and K. were my favorites).
Also, please check out E. L. Konigsburg, who has written some truly stupendous (and good conversation starting) books for kids. From the mixed up files of Mrs. Basil E Frankweiler is a classic, but there are a lot more that i loved as well.
My daughter is named after Madeleine L'Engle, who was my favorite author, and wrote a number of, perhaps young adult, novels. A Wrinkle in Time is a good place to start.
Terry Pratchet (after whom a son would be named) has also written some kids books (i just read Nation) that are thought provoking and hilarious.
Another idea for you is to see if Nancy Pearl has some good ideas through her website, or her Book Crush book (check it out via web site).
Also, to get your girls thinking about the stories behind the words, maybe it will be helpful to have a little 'question' time after the reading -- just one her two 'what did you think of', or, 'why do you thin the character ...', or 'what was your favorite part?'
What a wonderful thread. Thanks. I can't wait to see what other people remind me of, or suggest anew! Good luck.