The Thing about Jellyfish, by Ali Benjamin

Suzy Swanson is invisible to her peers. Which is maybe for the best. She knows a lot of facts that would get her relentlessly teased and she would be dubbed "the nerd" for the rest of her school days. For example, she knows that there are 150 million jellyfish stings around the world each year.

Once, she hasn't always been invisible to everyone. Once, she had a friend: Franny Jackson. The two were like chocolate and peanut butter. But then Franny met some other girls in their school and slowly became one of those snobbish, stuck-up girls who are obsessed with make-up, clothes, and boys.

Gradually, Suzy and Franny began to drift apart.

Suzy watched in despair as her best friend transformed from the girl who had spent countless nights having sleepovers with her, who had run around with her as the sky grew dark, who had been entrusted with her secrets, into one of the girls who they had always despised. 

Then, one day in the summer, Suzy is out in her yard enjoying the nice weather, when her mom calls out to her, telling her to come inside. Suzy can tell that her mom is very upset. Then, her mom says three words that cause Suzy's world to fall apart: "Franny Jackson drowned."

It's the beginning of seventh grade. Suzy is with the rest of her class at an aquarium. She is nostalgic thinking about Franny, when she notices the entryway to the jellyfish section. She glances back at her class, knowing that, despite the neon t-shirts they had been instructed to wear, none of them would notice her disappearance. Social invisibility has its advantages.

In the "Jellies" section, she sees a photograph of a jellyfish in a glass jar. The jellyfish is an irukandji, the most dangerous species of jellyfish in the world. It is small, nearly invisible, a little less than the size of a human thumbnail. Irukandji stings are fatal if not treated, and symptoms can easily be mistaken for a common illness if the jellyfish is not noticed. These facts lead Suzy to formulate a hypothesis: Franny Jackson's death was caused by the sting of a jellyfish. And she decides that she will spend whatever time it takes researching, thinking, and even trying to reach out to jellyfish specialists around the world to prove her theory. 

The Thing About Jellyfish is about a middle-school aged girl who is trying to overcome the grief that one has when they lose their best friend. Before Suzy and Franny split, Suzy could talk for ages about anything. After they split, Suzy decided that it was best to just... disappear. become invisible. But now she has a mission, and she finds her bravery. She becomes determined to solve the mystery of Franny's death... but can she?

There are many people who can connect with Suzy, whether they have experienced the loss or death of a friend, or just appear to be invisible to the crowds. I would recommend this book for confident readers of chapter books 8+ (might be a bit challenging for younger readers, but it is a lovely story), or anybody 10+.