What is considered the "book of the Law"?

Reading 2 Chronicles 34 this morning where Shaphan took the book to King Josiah and he read it to Josiah.

The king is also commanded to write himself a copy of the Law. I wouldn't think that the first 5 books where read to Josiah in a quick reading, so it has to be more specific. Does it possibly only consist of Exodus 20-23?

Does anyone have a better understanding of what this book specifically consisted of?