Lots of good stuff on Ruth.
Of first importance are Hubbard, Bush, Block, Morris, Younger, Lacocque, Sasson, Matthews, Nielson, Sakenfeld, and Campbell.
Of secondary importance are Farmer, Jackman, Murphy, and Atkinson.
Forthcoming is Korpel.
These are bound with Judges: Block, Morris, Younger, Jackman.
These are liberal: Lacocque, Sasson, Nielson, Sakenfeld, Campbell, Farmer, Murphy. There are no particularly good Jewish commentaries on this book.