Markovits's slim volume is the story of Polidori, Byron's doctor, most famous for writing The Vampyre at the same gathering that Mary Shelley was inspired to write Frankenstein. It's very tightly written, sensitive to the nuances of Polidori's emotions, and those of Eliza, a woman who mistakes him for Byron, and falls in love with him.
The imposture of the title is manifold. Polidori's story is passed off by his publisher as by Byron, which creates huge sales, but because of that Polidori cannot make a proper claim on the income from it. Polidori doesn't disabuse Eliza of her mistaken identification, although he intends to. The novelty of having a woman pursue him, having been intimidated by witnessing Byron's conquests, makes him hesitate, and allow the self-seduction. Polidori also passes himself off as a doctor - although trained as one, he's plainly not competent. The whole book is imbued with failure - Polidori has been too close to one so great, and measured himself by comparison, he's a poor doctor, lover, gambler, and even his literary success is stolen from him.
A friend who has a specific interest in Byron and his set got fed up with the inaccuracies of the book, but it is a fiction, just as Byron created fictions about himself. Perhaps such books are better for not knowing the facts behind them.