"My Enemy, My Ally" by Diane Duane is the 18th numbered novel in the Star Trek line published by Pocket Books.  This is a first-printing copy from 1984.

This is a used 37-year-old mass-market paperback.  It is in good condition: whole and complete, all pages intact and binding still tight.  It shows minor wear and tear along its edges, a small crease on the front cover and some creasing on its spine, which is typical for a book of its age.  It has been autographed by its author, Diane Duane, on the title page, with an inscription.