The book No One is Too Small to Make a Difference by Greta Thunberg is actually just a collection of Ms. Thunberg's speeches since 2018 about Climate Change. If you don't know who Greta is, or what she did, then this book may have less significance in your eyes because beyond a little half-page introduction, the book makes little attempt to introduce her.
Greta Thunberg is a young Climate-Change Activist who came to international renown when, at the young age of 15, she staged a School Strike for the Climate by picketing outside the Swedish Parliament to advocate for stronger action to be taken on Climate Reform. Her actions led to an international movement of students staging similar strikes to advocate for Climate Reform.
Reading about who Greta is, and what she has done (here's her Wikipedia page), and then reading her speeches, is enough to pique one's interest in Climate Change, and potentially lead one to inform oneself on the current situation. I know I did a bit of Googling after reading her book, and I hope you will too.