The book of Ruth is a very short book; just four chapters. It takes place during the same time frame as the book of Judges, but it must be towards the end, as it ends with the birth of Obed, David’s grandfather.
The book of Judges takes up where the book of Joshua left off. The Israelites are in the Promised Land, and they’ve started to take over the land, but they haven’t completed the job. Not every tribe has their land, yet.