This is really a 3 1/2. I really enjoyed meeting Tom, the lighthouse keeper, a man who could tell no lies. His attention to the light house and Janus is admirable. His career in the Army is heroic. He is a loving husband and father, though somewhat damaged son and brother.
I was shocked that Isabelle's mourning family so readily allows her to marry and live on isolated Janus, so far from them. Isabelle, from the beginning is a "wild child." I can see how her disappointments would make her very desperate, even crazed. I find it hard to believe that her family allowed her to go through these trials on her own. Tom has to stay on the island, not Isabelle.
Tom and Isabelle's life, their love, their care for Lucy, is described in beautiful prose. Yet, when Isabelle has to face the consequences of their bad, choice,her selfish and fantastic decision, the writing changes. Isabelle becomes like a lioness who is trying to hold on to her food. She is vicious in her attack of Tom. Lucy has been her meal for the past three years. 
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Because of Tom's sad childhood, he could not allow the widow, Hannah, to continue to grieve. Tom's self righteous father had exiled his mother because of an indiscretion. Yet Tom's need to tell the truth to his father and later to Hannah causes him exactly the pain he has tried to avoid.
The writing changes in Book III . Isabelle loses her wild innocence and seems like a coarse villain. Stedman's writing never fully recovers. Throughout, Tom, is The Light Between Oceans. He can illuminate the seas, but, he can do nothing for Isabelle, his nearest and dearest.

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