Obviously you are not asking for yourself,
. so I will raise my hand to answer.
Since to write good free verse, one needs to know most of what one needs to know to write metered verse, one should just go for a good book on prosody.
A good book for beginners who need a lot of handholding is Mary Oliver's "A Poetry Handbook." There is a chapter titled "Free Verse" which explains in the second para "Free verse is not, of course, free." It is a good book for scaredy-cats. (IMO.)
Better, and on a level for the hungrier novice is "Prosody Handbook: A Guide to Poetic Form" by Robert Beum and Karl Shapiro It also has a chapter on free verse that lets you know it ain't free. It is intelligently written and even poets who have been at it for considerable time will find much food for thought. Highly recommended.
Better, and bigger, the book that tells beginners (and lots of practitioners) everything they didn't know they should ask is "Western Wind: An Introduction to Poetry" by John Frederick Nims and David Mason. It is chock-full of excellent poems as examples. Every intrepid newbie just setting up poetic houskeeping should have a copy.
Obviously to write good (or even halfway decent) free verse, the poet needs to be able to use structure, rhythm, allusion, assonance, alliteration, anaphora, phrasal repetition, imagery, and lots more.
I have seen some pretty awful books on the subject too but the least said about them, the better.
I am awfully glad you put this question up, Michael, and I hope others will contribute other titles.
Crossposted with Maryann.