Technically a play not a book, but I read Miranda's copy. We will be going to see the play, too, so you may get a "play review" next year.
An Inspector Calls by J. B. Priestley is, as wiki says, 'classic "drawing room" theatre'. It works very well; well written, neatly handled, and there is one (or perhaps two) clever and one not so clever twists at the end. However there are flaws: the final ending is not needed and is one twist too many, and probably blurs the message; the inspector's final speech reads like something out of Ayn Rand (though mercifully much shorter) and is far too unsubtle; and the whole thing is, in a way, too well turned.
Let me do that last criticism first. The play is very "neat": everything fits carefully together, all the threads fit, everyone reacts as they have to. This helps keep things moving along and helps keep you reading, but begins to feel by the end a little too pat, a little too careful (yes I know my criticism of the final twist somewhat vitiates this point, but never mind, I shall leave you with it anyway).
The inspector's final speech is a rousing cry to, well, whatever; I didn't actually read it; or if I did, I immeadiately forgot it. I've read such things before. For an actual real inspector, it would have been totally out of character, and I think you'd have expected Mr Birling to notice and complain of this. But more importantly, it is entirely unnecessary to anyone who has been paying any attention to the play. The play has carefully brought out the various aspects of the way in which the family's behaviour has lead to the death of the young woman - or, when you realise she may have been a composite, the various aspects in which they have degraded the lives of various women. That's a good way to do it, because in a sense the audience "discover" this for themselves, and will be inclined to accept the ideas. But when the inspector jumps up and tries to ram the same thoughts down your throat, you rebel. Or at least, I did.
The revelation that the young woman is - probably - a composite is clever, and well done. Probably the best bit of the play. Suddenly, a number of things we saw before - principally, the inspector only showing a photo to one person at a time - all make sense. Also, excellently, we see the way different people react to it: most obviously, the two children retain their - deserved, admitted - guilt; whilst the parents discard it, concerned only for outside appearances. And Gerald is perhaps ambiguously in between.
A tension that I think could have been explored, but wasn't: what will the two children do? The implication, by the end, is that disgusted by their parents they will leave home, and live on... what? How will they balance their conscience against comfort and financial security? Another unexplored aspect was the relation between Gerald and Sheila. The implication is that their nice, comfortable but rather shallow "love" has been disrupted by events, but perhaps they have come to see each other more deeply; Sheila's brief note that they'll need to start from the beginning again is quite touching. Perhaps that's all that is needed.
I expected more about knighthood. At the start the scene is carefully set: Arthur is prosperous but second-rank, and the marriage and knighthood would pull him up. As the play went on he should have become more and more desperate to avoid any of this becoming public (he is concerned to some extent, but not nearly as much as he should be). Perhaps that wasn't necessary; the play is about something a little deeper than social class.
Lastly, the final twist - that someone really has died - rather jars. It seems pointless and disruptive. "What does that mean?" we inevitably ask, and inevitably there is no answer. Does it mean that the carefully constructed twist about the composite Eva is wrong? That would be most unsatisfying. Does it mean that instead of having to consider their morality, we're back to a rather more straightforward tale of worrying about nasty details becoming public? That, too, would be unsatisfying. Again, it seems so unsubtle; as though some theatre manager, unsatisfied with the play's existing subtle and ambiguous ending, wanted some zing at the end. Before that last twist, the "inspector" has come to seem like something supernatural, a kind of angel, who has brought harsh truth into these people's lives, and given them the chance to react and show their characters. That was all that was needed.