Once again, this seems like a logical progression from his previous book. One of the ironies in Artist Of The Floating World was that we're never sure quite how war-mongering Ono's art was: as long as he's in denial about his war responsibility, the other characters (who we only see through his eyes) seem to be blaming him for a lot, but once he admits his guilt, those around him act as if he has nothing to be ashamed of - they even laugh a bit that he thinks a mere artist could have had any influence at all.
Here, Ishiguro's again concerned with war guilt, but this time there's a figure who has been held unmistakeably guilty by his society: Stevens' employer Lord Darlington, who was a Nazi sympathizer and sometime anti-Semite engaged in actual treason against Britain, and who was exposed for same. No ambiguity about his guilt. Ishiguro's interest is still squarely focused on the issue of denial, but no longer is it the guilty party who's engaged in denial. Now he's examining how we can be blind to great moral failings in those we love, or look up to.
In that sense this makes a fine companion to Ishiguro's first two books; thematically they form a sort of trilogy, with thematic and tonal affinities that far outweigh the fact that this book takes place in a different country than the first two. The first time I read Artist I felt that its characters were Englishmen in disguise, and I still think that to an extent; certainly Ono and Stevens (and Etsuko's father-in-law) would understand each other.
This is the best of the three, however, and I think it's partly because in shifting settings Ishiguro has brought his themes into sharper focus. His main concern in these first three books is repression: of guilt, of emotion, of need, of self. In the first two books it seemed to me that he was locating this repression in an imaginary Japan constructed of equal parts Western stereotypes and Ozu mannerisms, and to me it felt a little overdone. In Remains Of The Day, he's fessed up to the fact that this repression (also) exists in Ishiguro's own England - but in locating it in Stevens the butler he's both hit on the perfect device for exploring repression (Stevens' view of the job description is that it's basically a professional represser) and managed to depict repression without stereotyping an entire culture. We're free to see bits of Stevens in ourselves and people around us (we'd be fools if we didn't), but at the same time Stevens is an extreme case, made believable precisely because his repression is depicted as a function of his vocation, rather than (solely) his nationality.
That's not the only reason this is the best of the three, of course. I'd also point to the fact that the England of Remains seems so much more vividly rendered than the Nagasaki of Hills or the unnamed metropolis of Artist: Ishiguro is simply surer of his setting here. Then there's the variety of tone in this book: it has the famous muted love story, which is undeniably moving, and it also has moments of humor the likes of which Ishiguro hadn't shown before.