With each book, the delightfully devious pre-teen heroine reveals something new about herself or her family's past. She was expelled from the Girl Guides. Like her mother, she's a genius. She still wears pigtails. She tries so, so hard to be an evil scientist but in reality is just a pre-adolescent who wants to hug hurting people all around her. She's morbidly obsessed with murder and death, but still believes in Father Christmas and concocts a brilliant plan to trap him in a chimney.
The literary nerd in me also loves Alan Bradley's series because each book includes so many references to English literature -- including two of my own favorites from my adolescence, "The Lady of Shallott" (Book 4) and "The Highwayman" (Book 5). The literary references are woven into the general narrative, so readers must be familiar with standard semi-famous lines from poems.
While the second and third books aren't as strong as the first, I think the fourth and fifths ones are. And the cliffhanger at the end of Speaking From Among the Bones has me eagerly awaiting Book 6's release early next year...