THE ELEVENTH SUMMER by Carlo Gebler (Dutton: $13.95). Carlo Gebler may be one of the most sensual fiction writers at work today. Though "The Eleventh Summer" is only his first novel, his sensibility seems fully matured, rounded, rich. The Irish countryside, where this childhood story is set, is evoked with such compelling detail that one can smell, taste, see, feel and hear it. The smell not only of dogs but of dogs' tongues, the prickliness of vegetation, the coldness of steps on bare feet: This is a real world full of sensations. In the novel, Paul, an 11-year-old boy whose mother has died, has been sent to live in the country with his grandparents. Granny, a woman broken by her daughter's death as well as her husband's wastrel ways, sleeps in the same room as Paul, and loves him completely. Grandfather is more wily, a sneaky old curmudgeon who bribes Paul to say he loves his grandfather best. Paul is a sensitive boy experiencing the world through the eyes of one on that delicate cusp between childhood and adolescence. The two major events of the novel, Paul's first abortive sexual experience with his cousin Philomena and the fire that ravages the grandparents' house (which Paul might have caused), are handled with great skill. Because these events are filtered through the prism of Paul's consciousness, they become completely extraordinary, exciting, terrifying. Paul is a wonderful narrator, completely believable because he is so earnest and smart and serious. The novel falters only when it waxes lyrical far too long without any incidents to hold the lyricism up.