For other than historic context of the Child Ballads, there's a good, old book titled Folklore in the English & Scottish ballads, by Lowry Charles Wimberly. It was published by the University of Chicago Press in 1928m and republished by Dover Publications in 1965. Used copies are readily available, but I didn't find a download version.
You can borrow the book for an hour at archive.org, but I'm not sure what it means to "borrow" an online book for an hour.

The Traditional Ballad Index gives good historical context information on songs, and I think it covers all 305 Child Ballads.

Steve Gardham mentions a Child Ballad Concordance (click) - but I think you want a commentary, not a concordance. Child died before writing a commentary on the Ballads, but he does have fascinating commentaries in the introductions of many of the ballads in his collection.