Complex sentence: Of these three, Granbury was the last to be completed, in 1969, and its proposed construction in the mid- 1950s became the impetus for John Graves' book, "Goodbye to a River".
Split into simpler sentences: Of these three, Granbury was the last to be completed, in 1969. When its construction was proposed in the mid- 1950s, John Graves wrote the book, "Goodbye to a River".