Lungeing and long reining by Jennie Loriston-Clarke. I picked it up second hand a while ago when I was doing long reining to help bring my boy back into work after injury and found it quite an interesting read.
Lessons on the Lunge For Horse and Rider by Mollie Sievewright, pub 1996, available second hand on Amazon. Covers long reining as well as lunging. It's very practical and has good photos.