I do not have a copy of this book. It seems to be the most difficult of Viertel's novels to locate. The title seems to be a reference to the military term, "line of departure," that is, the line made up of landing craft still at sea, awaiting orders to go to the beach to commence an assault on an enemy.
In a phone conversation with Mr. Viertel, I asked him about this book. He replied, "It has a weak story-line. Not that any of my books are none-too-strong."