Ariel by André Maurois — A biography of Shelley, originally published in French in 1923 and still one of the most readable accounts of the poet's life. The final chapters, covering Shelley's time at San Terenzo and his drowning in the gulf, are vivid and affecting. It reads more like a novel than a biography, which is both its strength and its limitation.
Mary Shelley: A Biography by Miranda Seymour — The fuller story of the Shelleys' time on the Gulf of La Spezia, with detailed accounts of the summer of 1822, the Villa Magni, and the aftermath of Percy's death. Seymour is good at rendering the coast itself — the particular quality of the light and the water that made the bay so compelling to the Romantics.
Italian Hours by Henry James — Not specifically about Lerici, but James's essays on Liguria and the Italian Riviera capture the way this coast felt to northern European visitors in the 19th century: sun-bleached, slightly other-worldly, shot through with literary associations. The chapter on Genoa is the best entry point.