Medieval tales effectively retold; Medieval Legends, adapted and edited by Philip S. Jennings. New York: St. Martin's Press. 191 pp. $16.95.

For adults whose main contact with medieval lore consists of the story of King Arthur, ''Medieval Legends'' is intriguing entertainment. These tales - translated into modern English - reflect the issues of their day, dealing with such subjects as courage, love, strength, weakness, fidelity and the occasional lack of it, hypocrisy, and the foibles of humanity and its institutions.

The first recorded version of the Hamlet story (some 400 years before Shakespeare's version), further antics of Merlin, and other not so familiar tales of Sir Gawain and Percivale are included. Then there is ''La Belle Jean,'' the story of a faithful wife unjustly accused of adultery, who steadfastly proves her love for her husband.

Since the advent of printing didn't come until the latter part of the Middle Ages, legends were passed along by word of mouth. The vivid imagery and colloquial ''Let me tell you . . .'' format make it easy to imagine what fascination and enjoyment these tales must have evoked when first told. And the color plate illustrations from medieval paintings, tapestries, and manuscripts add a strong note of authenticity to the tales.

Be advised, however, that these are not legends for children. In movie terms, the book would be rated at least ''PG,'' with some stories dealing with lasciviousness, though in a fairly restrained fashion.