I've completely lost count.
Big ds chose a new Magic School Bus book at a used book fair yesterday, his nose has been stuck in that a lot. I saw him picking up a Magic Treehouse book to read on his own accord, twice! That had me doing a happy dance! It is well within his reading ability, he's a very good reader, but I've yet to see him reading anything but picture books for enjoyment. I carefully steered his younger sibs away from him and didn't call him to the task he'd previously been asked to do for a good 40 minutes while he sat there flipping through it. He's also grabbed a few smaller books that are largely print with very little pictures, and read them instead of putting them back in the shelf. Had me very pleasantly surprised when he started telling me tales of Peter Rabbit, a book he's always ignored.
Big dd has been seen in a Word Bird book, a nursery rhyme book, The Golden Book of Animals, a Barbie something from the book fair, and she attempted reading a few board books to her very busy little sister.
Little ds can't get enough alphabet story books in, he got a Tonka tractor book with wheels on it from the book fair, every Thomas book in the house has been lugged under his arm throughout the week.
Little dd has gone through every board book in the house I think.