Across the River and into the Trees, published in 1950, is about death - a dying American colonel visits the scene of his first world war battles three decades earlier, and also remembers his recent love affair with a teenage Italian girl. It's been overshadowed by The Old Man and the Sea, which came out two years later and was the last novel Hemingway published in his lifetime, but I found it a moving elegiac (and fairly short) piece - I was a little surprised that it did not date from closer to Hemingway's own death. The two things Hemingway does well are the sparse prose depicting his male characters' interactions with each other and with women, and his descriptions of the story's settings, and those are very much in evidence here. Apparently it crashed and burned with the critics when first published, though it has regained favour since.
Islands in the Stream was written at about the same time, though not published until 1970, nine years after Hemingway's death. It's a longer and actually less successful book, in three parts, of which the best is the first, set in the Bahamas where the central character, an exiled American painter, takes pleasure in the visit of his three sons. There is a particularly vivid passage where one of the boys is locked in battle with a swordfish, apart from the age of the fisherman very reminiscent of The Old Man and the Sea (published 1952 and possibly reworking some of this material). The two other parts are set in and around Cuba during the second world war, the second section mainly in a bar, dealing with Cuban politics and women; I thought at first that the third section, in which the protagonist hunts the crews of wrecked German U-boats, was a bit far-fetched, but was surprised to learn that Hemingway actually did this for real. Altogether, it didn't seem to me to have the same oomph as the works I have read published in his lifetime.
Obscure books that Hemingway owned which I also have read:
William Heinemann, a memoir by Frederic Whyte
Starling of the White House by Edmund Starling and Thomas Sugrue
The Master by T H White
Esprit De Corps by Lawrence Durrell
The World of Washington Irving by Van Wyck Brooks
The real Charlotte by Somerville and Ross
A narrative of the life of David Crockett, Of the state of Tennessee by Davy Crockett
The Flowering of New England, 1815-1865 by Van Wyck Brooks
Elizabeth and Essex: A Tragic History by Lytton Strachey
Eastern Approaches by Fitzroy MacLean