Try buying them an e-reader like a Kindle. But why encourage them to simply read? For the sake of reading comprehension or for absorbing knowledge from the content? You may be able to encourage them to learn using multimedia mediums rather than books. Youtube has a lot of great shows that explain concepts in math and science. Khan Academy (http://www.khanacademy.org/) is also a great video-learning resource.
I do not have pre-teen girls and it may be years before I do. But personally I like to read, ever since a child. I think you can read books with your girls together, and share the reading experience with them. Better you can try reading the same books, though they might be too simple for you:)
Take them to the bookstore and allow them to pick out books that actually interest them. They will be more likely to read a book that interests them and that they had a hand in picking. Also you can try instilling a routine of reading for about 30 minutes before bed time, that's how my parents got me into reading. I now can't fall asleep with out reading becuase it has become such a routine.
One of the best things you can do to encourage your kids to read is to read yourself. They need to see you reading to value it themselves. That said, you may end up with a daughter like mine who "hated" to read. The problem is that her older sister is a heavy reader with a penchant for fantasy novels. When she perused that collection, it had no appeal to her. What we started finding books that would appeal to her and she became more interested. We also read a chapter together each night in bed, aloud. Yes, even at age 10 1/2. Then she goes on to read a little bit more herself. I read almost all the books she does, so we can have a conversation about the characters and what is happening. She likes having something for us to share, which is really important. You may get tired of reading horribly written series, but what's important is to establish the behavior. One other thing you might do is get them reading things that don't really seem like books - specifically graphic novels. There's a new one called Smile that is a good book for girls. My daughter also loves Calvin and Hobbes cartoon collections. Even though it may not seem like reading, it helps to make it a habit. Finally, so many books are made into movies, you can suggest reading the book before going to see the movie. See what happens when your kids can compare what they read and what they saw. It's a good motivator when they realize they can spot plot discrepancies - almost like a game. Good luck!