THE BOSS DOG by M.F.K. Fisher (North Point Press: $16.95; 118 pp.) If you really love M.F.K. Fisher-- and you're a "dog person"--you'll like "The Boss Dog"; it's the story of one of those cute mutts with endearingly human attributes. There is much charm in this slim novel about an American mother and her two little daughters who spend a year in Aix-en-Provence. Fisher's prose is delightful when she is describing the many attractions of this French city, and true to form, her descriptions of food are simply delicious. She captures the impatience of American children for the long, drawn-out, intense pleasure of Carnival that goes on for days, and their jealousy of the French children whose time and place this is. The dog, it seems, bridges the gap between the little family and the people of Aix; in their anxious sense of being outsiders, the three Americans attribute to the dog all the human qualities they long for. Yet Aix is the place where their lives are richest; it becomes home. The dog seems to have been just a floppy ear on which to hang a tale.