Okay, two things: this is one of my favourite books that I have ever read; I am astounded by some of the poor reviews here!
Contrary to other posters' experiences, 'The Island of the Day Before' was the first Eco book that I read, and what instantly grabbed me was his fantastic style. I know that we're reading it in the English translation (see his book of essays 'Mouse or Rat: Translation as Negotiation'), yet the prose unfurls and unfolds mesmerically, drawing the reader into the novel. The narrator's tone is engagingly learned, affectionate towards his characters, and very, very funny.
Then there's the characterisation. Roberto della Griva himself is such a brilliant creation: a sub-standard Petrarch trapped on an abandoned ship writing letters to the love of his life who doesn't even know he exists; an unwitting witness to some of the greatest occurrences of his age; a figure who lays bare the mixture of disillusion and enduring hope of the human existence. And, of course, we must not forget Father Casper...
So now we come to the brilliant plot, or, perhaps, plots is more accurate. I really don't understand why some reviewers here have said that nothing happens; if anything, there is too much happening, with the flashbacks and the background detail, the stories of warring regions and the conspiracies of Cardinal Richlieu. This is as much of the story as the actual 'present' of the novel. And all these interesting and revealing episodes are framed within each other, creating a fantastic richness and depth that really draws one in.
This is really Eco's most honest novel. I can't agree with those who have labelled it especially intellectually ostentatious. In his other novels Eco can cloak his erudition and intelligence, in a way. In 'The Name of the Rose', for example, it is all wrapped up in a detective-like structure, so it really doesn't matter if all the allusions aren't noticed, or the minor details understood: by the end, it all comes together. Here, however, these reflections aren't just asides, but intergral to the novel. To say it's seld-indulgent or pretentious is completely missing the point: it is simply and completely genuine, and unashamedly so.
It is a novel of reflections; just as Roberto reflects on his life, his past and his love, what it means and where he can go from here, whilst he is trapped aboard the ship in solitude.
Buy this, read it in one go, and simply reflect on it all...