1. Block is on top! Daniel Block's commentary on Ruth in the NAC series is THE best resource on Ruth. I cannot speak for the Judges portion of this commentary, but the Ruth exposition is second to none. This is the most thorough and clear commentary on Ruth I read, and it is wonderfully Christ-centered! If you don't want to take my word for it, here are some other voices (much more helpful than mine):
-Tremper Longman gives this volume a 5-star rating.
-Desiring God lists this as the best commentary on Judges/Ruth.
2. Faithful Indeed. Sinclair Ferguson's small little book called Faithful God is a faithful exposition of Ruth. This is basically 4 messages that Sinclair preached on Ruth in book form.
3. Love this Title! This is a brand new book in the Gospel According to the Old Testament series (see other volumes in this series here). From Famine to Fullness: The Gospel According to Ruth by Dean Ulrich is a great meditation on Ruth. The title of this book accurately communicates the message of Ruth.
4. Duguid Good. The Ruth section of Esther & Ruth in the Reformed Expository Commentary series is very helpful. It contains 5 sermons in written form from a great Old Testament scholar: Iain Duguid.
Here are some other commentaries I read but did not find much help from (which is more of a reflection on my weaknesses than the authors'):