I found the set-up very familiar when I started this-- the whole girl-in-a-cellar thing reminded me of Silence of the Lambs and countless others. But I suppose it was a fresher idea when this was written in the 60s.
Also, once I got onto Miranda's part of the book, I realised that apart from being a psycho thriller, this book is really about the struggle between the aesthetic and the anti-aesthetic. I am totally married to a 'Fred', who just doesn't get how I find ugliness dispiriting and I can totally identify with Miranda's feelings of not wanting to seem snobbish about Frederick's lack of taste but being unable to suppress her own yearning for beauty. At the end, what I found even more tragic than her death was that he read her whole diary and still didn't 'get' what she had been trying to show him about beauty.
I liked that the two povs weren't interwoven; it allowed you to become totally immersed in each of their stories without distraction. The end is chilling, again, I think Fowles is trying to say something greater about his time when Frederick's outlooks 'wins'.