The Glass Ocean by Beatriz Williams, Lauren Willig, and Karen White

In 1915, wealthy Caroline Hochstetter is sailing to Europe with her husband, Gilbert, on the Lusitania, even though there is danger that  the ship could be sunk by a German U-boat. Also on the ship is a young woman named Tess, who is working in tandem with her older sister, Ginny, to secretly copy an unfinished waltz by Strauss that is owned by the Hochstetters. Tess comes into contact with an old friend of Caroline's, Robert Langford, while observing Caroline on the voyage. Robert has been in love with Caroline since they first met and Caroline has feelings for him, too, but she also loves Gilbert, even though Gilbert has been distant lately and feels it's  his duty to shield Caroline from his business dealings and any unpleasantness going on in the world. Meanwhile, in present time American writer Sarah Blake is desperate for a good idea for a new book. She decides to travel to England to contact Robert Langford's great-grandson, John, to see if she can look at the Langford family archives. Sarah's great-grandfather, Patrick, was a steward on the Lusitania and Sarah has a watch that was given to Patrick from Robert all those years ago. In The Glass Ocean, the reader learns what happened to Caroline, Tess, and Robert on that fateful crossing through their eyes and from the contemporary research of Sarah and John. An enjoyable read for those who like historical fiction.