The Lusitania Saga and Myth: 100 Years On [Ramsay]
All this ended on 7 May 1915 when she was torpedoed by a German U-boat and sank 18 minutes later with the loss of 1,198 passengers and crew (interestingly 39% of those aboard survived whereas only 32% of those on the Titanic survived despite the latter taking 2 hours, 40 minutes to sink.)
The Author concentrates not just on the disaster but its aftermath including the political recriminations and the inquiry. As a result of the loss of 128 American citizens the Germans signed an agreement not to attack US shipping. Their breach of this was a major contributory reason, along with the Zimmermann Telegram, why the USA entered the War.