I'm getting a lot of questions about Ancient Rome that I cannot answer and my kid has a new Kindle sitting under the tree.
Non-fiction.
I, Claudius is fiction, but a good start.
A great source for a teen is Larry Gonick's The Cartoon History of the Universe (specifically, volume 2: https://www.amazon.com/Cartoon-History-Universe-Volumes-8-13/dp/0385420935).
It's accessible and funny, but it's also carefully done.