I'm posting my thoughts without reading (yet) much of what has gone before, so as not to be overly influenced by the opinion of others. I reserve the right to change my mind later

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Putting aside the subject matter for a moment, I was firstly struck by how beautiful some of the writing is. The start of Chapter Two, for example:
"A brilliant day, hot, with a brutal staring sun pouring down rays that were like molten rain." I love that image of the sunshine. There are various other examples that I won't repeat here.
As to the story - I have had my horizons broadened by this tale. I was aware that there have been cases of white skinned people being born into black families (and vice versa), but I was surprised by the seemingly common practice of "passing" as put forward in this book. Were there really that many people living a lie at that time? And I find it a bit difficult to believe that they wouldn't be "found out" eventually, unless they completely severed their roots and moved away from their home town. As I say, this is new to me and I'm happy to be convinced either way.
I thought the fact that there may have been an affair going on between Clare and Brian, Irene's husband, was telegraphed pretty early on, and was surprised how long it took Irene to come to the same conclusion. May have been in denial, I suppose.
I found the ending somewhat abrupt, and would have liked there to been more detail on the subsequent fallout of Clare's death. And as far as who killed her? I'm with Catlady on this one. I think that Irene, on the spur of the moment, took advantage of the situation and "assisted" Clare out the window. At least, that was my initial impression and for now I'm sticking with it.
And, just as an aside, I noticed something that I'm not sure I've ever seen before, brackets used in speech:
"I'm sure I am. Completely. (Except, of course, when it's just patronizing kindness on their part.) And I know colored girls who've experienced the same thing--the other way round, naturally." -
Irene speaking in Chapter Three.