Complex sentence: It was she who lent him "The Progress of Romance" by the Gothic authoress Clara Reeve in which he found the story "The History of Charoba, Queen of Egypt" that inspired his poem, "Gebir".
Split into simpler sentences: It was she who lent him "The Progress of Romance" by the Gothic authoress Clara Reeve. In this he found the story "The History of Charoba, Queen of Egypt", which inspired his poem "Gebir".