When and by whom the book of Ruth was written, are points not agreed on among critics and commentators.
As to the transactions recorded in it, they are variously placed. In the book itself there is no other notation of time than merely this, that the things came to pass in the days when the judges ruled; therefore some have placed these transactions under Ehud; others, under Gideon; others, under Barak; others, under Abimelech; and others, under Shamgar. This last is the opinion of Archbishop Usher; and most chronologers adopt it. The book is evidently an Appendix to the book of Judges, and contains a perfect history in itself; and therefore should not be inserted in any part of that book. It also seems to be an Introduction to the books of Samuel, in which the history of David is contained, as it gives the genealogy of this prince. It is also not without its use in matters which respect the Gospels, as it ascertains the line by which Jesus Christ came.
As to the author, he is as uncertain as the time. It has been attributed to Hezekiah, to Ezra, and to Samuel; and it is most likely that the author of the two books of Samuel was also the writer of this little book, as it seems necessary to complete his plan of the history of David. See the preface to the first book of Samuel.
The sum of the history contained in this book is the following: A man of Bethlehem, named Elimelech, with his wife Naomi, and his two sons Mahlon and Chilion, left his own country in the time of a famine, and went to sojourn in the land of Moab. There he died; and Naomi married her two sons to two Moabitish women: Mahlon married Ruth, who is the chief subject of this book; and Chilion married one named Orpah. In about ten years both these brethren died; and Naomi, accompanied by her two daughters-in-law, set out to return to the land of Judah, she having heard that plenty was again restored to her country. On the way she besought her daughters to return to their own country and kindred. Orpah took her advice, and, after an affectionate parting, returned; but Ruth insisted on accompanying her mother-in-law. They arrived in Bethlehem about the time of harvest; and Ruth went into the fields to glean for their support. The ground on which she was accidentally employed belonged to Boaz, one of the relatives of Elimelech, her father-in-law; who, finding who she was, ordered her to be kindly treated, and appointed her both meat and drink with his own servants. Finding that she was by marriage his kinswoman, he purposed to take her to wife, if a nearer kinsman who was then living should refuse. He was accordingly applied to, refused to take Ruth, and surrendered his right to her, according to the custom of those times, at the gate of Bethlehem, before the elders of the city. Boaz then took her to wife, by whom she had Obed, who was father to Jesse, the father of David.
To the questions, Who was Boaz? and, Who was Ruth? no satisfactory answer can be given: all we know for certain is, that Boaz was an Ephraimite of Bethlehem; and Ruth a Moabitess, and consequently educated a heathen. But what we want in certainty several have attempted to supply by conjecture; with them Boaz was the same as Ibzan,Jdg 12:8-10; and Ruth was the daughter of Eglon, king of Moab. This is the opinion maintained by the Chaldee Targum on this book; to which I shall, in the course of the notes, have farther occasion to refer. The rabbins say that Elimelech was brother to Salmon, who married Rahab; and that Naomi was his niece.
The genealogy of David, as stated in this book, is as follows: -
This chronology is according to Archbishop Usher; and includes, from Judah to David six hundred and seventy years.
Taken from "Adam Clarke's Commentary on the Bible" by Adam Clarke, LL.D., F.S.A., (1715-1832)