This is the 60th most frequent Chinese word.
里
"里" means "inside," "in," or a unit of distance (~500 meters).
In this sentence, '里' indicates a location within an object (the book). It specifies that the stories are located inside the book.
这本书里有很多有趣的故事。
This book contains many interesting stories.
Here, '里' is used metaphorically to denote an internal state or condition (inside one's heart).
你心里在想什么？
What are you thinking in your heart?
In this case, '里' is used to express a physical location within a container (inside the bag).