Captain Corelli's Mandolin by Louis de Bernières (1994) is the obvious starting point — the novel that put Kefalonia on the map for international tourists. Set during the Italian and German occupation of the Ionian Islands in World War II, it follows a love affair between an Italian officer and a local woman against a backdrop of occupation, massacre, and the chaos of Italy's armistice. The island's landscape and community are central characters. Read it before you go rather than after; you'll visit Fiskardo and Myrtos differently.
The Island of the Blind by Louis de Bernières (2023) returns to the same landscape decades later, a more elegiac meditation on memory and loss. Less essential than Corelli, but worthwhile if you're already taken with the setting.
The Ionian Vision: Greece in Asia Minor, 1919-1922 by Michael Llewellyn Smith gives essential context for the Greek national story — the catastrophe of the Anatolian campaign shapes Greek identity and the Ionian Islands' relationship to the mainland in ways that echo through local history.