Spies in Sandals [Book Four]

***Book four in a series, please read the others first!
After the whirlwind that was last winter, Nat thinks she's evolved out of the spy world. It's been eight months, after all, and it's hard to think about spies when it's August and school is about to start. So she focuses instead on enjoying her summer and the occasional presence of a super-hot, still-super-mysterious boy.
Nat thought she had finally figured out Zach Sullivan during the blizzardy winter. But it's during the heat-drenched days of summer that the truth comes out about something that only ever existed as a rumor in CIA files: Zach's mother.
What happened to Zach when he visited home in July? What caused the CIA to suddenly jump back onto a rumor they'd left alone for almost ten years? Who or what exactly is Zach's mother?
Everyone has questions about her, and the only place that could have the answers is Boston, home to Harvard and the Red Sox and generations of Zach's family. So Nat finds herself walking the cobblestone streets of the North End, strolling through the Boston Commons, and wading in Cape Cod as she fights between her loyalty to Zach and the creeping doubt that he will always be hiding something from her.
And then the biggest surprise hits...and it's up to Nat to fix it.
Spies in sandals? The heat is on.