This book is written by the first president of the Lutheran Church - Missouri Synod. Well, technically, he spoke weekly sermons to seminary students over the course of a year and they wrote it all down. So that is something to know about this book: it is a series of lectures to seminary students.
This book is like finding the key that unlocks Bible interpretation. In short, if a Bible passage instructs *you* to do anything (honor your parents, take up your cross, rejoice in the Lord) then that passage is Law. There is nothing wrong with that! The Law is good. We should keep and obey them. But they cannot save us.
If you read a Bible passage and it describes something *God* did for you (Jesus dying on the cross, take eat this is my body given for y0u, the gift of the Holy Spirit given in baptism, saved by grace through faith) then that passage is Gospel. We receive the Gospel through no merit or worthiness in ourselves. It is a free gift which me must accept to be saved.
The book proceeds to describe the many, Many, MANY ways people, especailly preachers, conflate the Law and Gospel and thereby destroy (or at least muddle) the Scriptures.
I recommend this book to anyone that is interested in rightly dividing the Word of Truth. But keep in mind these are lectures to Lutheran seminary students in 1880s.