3.5*, mostly for the third section, and for the potential that the first and second sections didn't quite reach.
Throughout the second section, and to an extent the third, I felt there were intended connections that I should have been making that I was failing to see. By the 4th section though, particularly with its disappointing, featuring out nothing of an end, I concluded that those connections weren't there at all. I was very much hoping that the fourth section would link us back to the first, and that (without spoilers) we would learn something about the author of the first section, but that didn't happen, Which feels like a missed opportunity.
Looking back on the whole thing now, I feel that this is a book that it's author wanted to be his version of AS Byatt's Possession, but it didn't make it.