by Peter Ackroyd. Grove, $17.95. Mr Ackroyd’s characters include an ailing poet, a dotty novelist, and a homosexual couple who refer to each other as “she” in defiance of gender. There are some fairly ordinary types involved as well, but they, while amusing, are peripheral to the author’s purpose. He is discussing, in fictional form, the line that separates imaginative invention from deliberate deceit, or emulation from plagiarism. Chatterton, that amazing eighteenth-century forger of medieval poetry, is not exactly a character, but his shadow falls over the whole engaging, clever tale.