A whole lot of fuss is made about secrets in this book. Mrs. Frankweiler hoards them (along with her art collection), Claudia seeks them, and Jamie is determined to help his sister find one.
So what's the big deal with having a secret, especially if it has to do with something as academic as a piece of artwork? It's not even a juicy secret, like who Claudia has a crush on at school. Well, secrets are important because they change the way that you think about yourself. As old Mrs. Frankweiler puts it,
"Secrets are the kind of adventure she needs. Secrets are safe, and they do much to make you different." (9.235)
Secrets are "safe" adventures because they don't involve doing crazy things in order to get a thrill. You don't need to skydive, or drive backwards really fast, or even try the cinnamon challenge (no really, don't do it) in order to feel like you've got an edge. Instead, secrets stay inside you and give you a thrill because you are the only person who knows them… and that's what makes you special.
For Claudia, the secret of the angel is what keeps her going, what gives her life purpose. Mrs. Frankweiler feels the same way, and it's even more important for her to collect secrets because she's old and needs things that can continue to pique her interest and give her something to live for. She can feel like she's dangling some juicy information over people's heads… and that's got to make her feel young (and a little devious).
That's why she doesn't tell the museum officials that the angel statue is made by Michelangelo, and that's why she doesn't tell Claudia and Jamie that their grandfather is Saxonberg. She enjoys keeping secrets to herself far too much.
P.S. Is this a good message for a children's book? That secrets are good? What ever happened to "secrets don't make friends"?