This is a tale of two girls who become spies in the French Resistance to German occupation during WWII, part ways after the war, and find each other again through a granddaughter after Hurricane Sandy in New York.
Lily's grandmother shoves a pen into her hands as she's being evacuated from her nursing home during the flooding. Lily promises to keep it safe, but she's 12, and she promptly loses it. Amidst the turmoil of the evacuation, she finds a pen dealer who has a long-standing order for the exact pen (down to the engraving!) she lost, and in fact, has already sold it.
Lily tracks down the buyer and finds so much more: a story of WWII, the French Resistance, and her grandmother as an intrepid spy.
Despite the horrific WWII setting, Skylark and Wallcreeper is a very gentle MG book.