Book talk is like when someone is telling you about a really cool toy they played with or a yummy snack they ate. Only instead of a toy or a snack, they're talking about a book they read.
The person doing the book talk might tell you the title of the book and who wrote it. Then, they might give you a short summary of what happens in the story--kind of like telling you what a movie is about, but without any pictures.
When someone does a book talk, they're usually trying to convince you to read the book yourself. They might tell you what they liked about the book and why they think you might like it too.
Sometimes book talks happen in a classroom or library, and the person doing the book talk might even have copies of the book for you to check out and read yourself.
So if someone ever says "let me tell you about this awesome book I read," just listen up--they might be doing a book talk!