I do not know if any copies of the book are still available.
It was produced by the Genealogical Society of Desoto County and I tried their web address (which was hosted here on Roots.web) but it said the page was not available.
Their postal address is P.O. Box 607, Hernando, MS 38632-0607. Their library is located on the south side of Hernando on Hwy. 51 in a building that is part of the government agricultural complex. It used to be open only about three days a week and they would answer the phone when open. Very helpful volunteers there.

I got my copy by "pre-subscribing" when they were seeking information to include in the family history section. I was a member of the Society at that time, but alas, I have let my membership expire.

I note that you are a Johnston, as am I. Which family?

My line comes from Thomas Henry Johnston (b. 1832 KY) and Anna Maria Jeter. He had followed an Uncle to Hernando, William Henry Johnston (1815 KY) 1m. Cynthia Ann Jones; 2m. Agnes Pryor Isaacs.

Some other Desoto Johnston families that I can not connect to my line are:
Joseph Johnston(b. 1786 VA) m. to Mary Darden.
Samuel Johnston m. Minerva Leonard.