I See London, I See France by Sarah Mlynowski

I see London, I see France, I see Sydney’s underpants.

Nineteen-year-old Sydney has the perfect summer mapped out. She’s spending the next four and half weeks traveling through Europe with her childhood best friend, Leela. Their plans include Eiffel-Tower selfies, eating cocco gelato, and making out with très hot strangers. Her plans do not include Leela’s cheating ex-boyfriend showing up on the flight to London, falling for the cheating ex-boyfriend’s très hot friend, monitoring her mother’s spiraling mental health via texts, or feeling like the rope in a friendship tug-of-war.

As Sydney zigzags through Amsterdam, Switzerland, Italy, and France, she must learn when to hold on, when to keep moving, and when to jump into the Riviera…wearing only her polka-dot underpants.
 
- Goodreads Summary