Can someone tell me where I can find this book? --And Then There Were None) /Agatha Christie
You’ll need to purchase it legally, since And Then There Were None is not in the public domain and remains under copyright until 2035. Fortunately, it’s widely available online from various retailers, both in digital and physical formats, so you can choose whichever you prefer.
Here is one such example of availability:
https://www.amazon.com/s?k=and+then+there+were+none+by+agatha+christie