Complex sentence: He married his third wife, Opal Laurel Holmes, in 1940, and she was his co-author on "Gold Rushes and Mining Camps of the Early American West" (1968).
Split into simpler sentences: He married his third wife, Opal Laurel Holmes, in 1940. She was his co-author on "Gold Rushes and Mining Camps of the Early American West" (1968).