Complex sentence: Melville briefly became a schoolteacher before he first took to sea in 1839, as a common sailor on a merchant voyage to Liverpool, the basis for his fourth book, "Redburn" (1849).
Split into simpler sentences: Melville briefly became a schoolteacher before he took to sea in 1839. This voyage to Liverpool as a common sailor on a merchant ship became the basis for his fourth book, "Redburn" (1849).