The Body in the Library is a mystery novel by Agatha Christie, first published in 1942, sez wiki, but don't read that, or indeed this, unless you want spoilers. I enjoyed it; it is pleasingly baffling but in the end nicely resolved.We start with a dead girl, or rather woman, Ruby a dancer, found in the library of Colonel Mustard. Eventually it turns out that it is even in the wrong library, which is witty but I think not quite plausible. After some investigation occurs we have a second dead body, in a burnt-out car in a quarry, this one a schoolgirl. Eventually - I think I realised this just a little before Miss Marple reveals it - we discover that the two bodies have been swapped; this neatly links the two apparently unrelated deaths, as well as invalidating all the carefully contrived alibis since the time of death is no longer what it was.
The cost of this is to make the evil culprits able to murder a perfectly innocent schoolgirl - for no reason but alibi -, on top of the dancer-come-golddigger Ruby, who is more plausibly murderable. That seems quite a lot to ask. Note that by the use of language we are lead to think of one as a woman and another as a girl, which makes it less likely in our minds that they could be swapped.
Once Miss M has worked out the swap, it would be natural to get the girl's parents to ID "Ruby"s body; this inexplicably isn't done, but would have made all the police work a good deal simpler. Ditto, I think, re the bitten fingernails; it was remiss of Miss M not to share this earlier.
Oh, and also: Josie, staring at the body of "Ruby", radiates anger. But - as she well knows - it isn't Ruby, it is Pamela, so there is no cause for anger, only sadness.
