I've been meaning to read this for ages, and have finally plucked it off my bookshelves. What a great story! It's unsettling and atmospheric, and I like the sense of is-it-real-or-is-it-not that the author promotes. I think what I like best of all, though, is that there's no solution. We never find out what happened to the missing girls. I understand from Wikipedia that there was a chapter, sensibly taken out by the publisher before publication, that gave a somewhat science-fictional explanation. Honestly, the total ambiguity and sense of mystery just works better as is. It's also what tips it over into horror in my opinion; mystery stories often have solutions while horror stories are less reliable that way. 

The ending, I have to say, didn't quite work for me as much as the rest. It was a little too abrupt, and I thought the conclusion of the Sara storyline was a little too.... concrete, I think I'd describe it? Although the Wikipedia summary gives an entirely different interpretation of Sara's fate than what I thought, so perhaps the ambiguity is there as well. That said, the whole was still wonderfully creepy, and I'll certainly be reading it again in the future.