This little book is, as it were, an appendix to the book of Judges, which the Hebrews generally make one and the same with the book of Ruth, and that not incorrectly; for this history happened in the time of the Judges, Ruth 1:1 (Bonfrerius). It is one of those little books that the Hebrews call חָמֵשׁ מְגִלּוֹת, Hamesh Megilloth, the five scrolls (Vatablus). Some of the Hebrews reckon it among the Hagiographa (which are the Song of Solomon, Ecclesiastes, Ruth, Lamentations, Esther). Others were joining it to the book of Judges (Drusius). It could be inserted between chapters 3 and 4 in Judges (Lightfoot). The Writer of this little book is thought to be, either, 1. King Hezekiah; thus Petrus Comestor[1] (Serarius on Ruth 4): or, 2. Ezra; to whom several other books are also attributed (certain interpreters in Serarius): or, 3. Samuel (Serarius, Bonfrerius, Drusius out of Isidore and Eugubinus,[2] Tostatus, Rabbis in Serarius); for by the consent of all he began the books of the Kings, and so it is likely that he was not willing to omit the narratives concerning the origin of King David (Serarius). They think that this book was published principally for this, that the genealogy of Christ might be woven in, who according to the flesh was going to descend from David[3] (Vatablus, Bonfrerius). For, the whole of Scripture has regard to Christ as its end (Bonfrerius).
[1] Petrus Comestor (died c. 1178) served first as one of the Chapters of Notre-Dame, and then later as the chancellor of its theological school. He is most remembered for his Historia Scholastica, a history of the world from the creation to the Acts of the Apostles.
[2] Augustinus Steuchus (1496-1549) was an Italian Roman Catholic scholar, who served as a prior of the Canons Regular of the Lateran, the bishop of Kisamos in Crete and prefect of the Vatican Library. He brings his varied talents in languages and antiquities to bear upon exegesis.
[3] Romans 1:3.