I just read a book called “Fermat’s last theorem” by Simon Singh, and I think it was awesome. I actually listened to the audio version of it. The book does not assume any prior knowledge of mathematics, and a person in 8th or 9th grade should be able to easily follow it.
I also just finished listening to an audio book called “love and math” by Edward Frenkel, and it’s been great. Nonetheless, it’s written by Edward Frenkel, so nothing less is expected. The book is a short biography of the author and his work, with a lot of connections to the history of those parts of mathematics related to his book. A lot of times Frenkel clearly defines some basic mathematical notion and gives lots of examples, but also there are numerous times that he starts talking about some of the most complicated topics in mathematics without giving much details about it. To be honest, the book is not a place for those details anyway. However, one of the nicest things about his writing style is that he repeats many concepts over and over, in order to make the reader feel comfortable with the mathematical notions he is using in the book. Another important property of the book is the great mixture of mathematics with the real life stories. It keeps both a mathematician and a general reader entertained. Furthermore, the beautiful viewpoint that the book provides for a professional mathematician as well as for a mathematics enthusiast is priceless.