Strange happening in the House of Smat

A book has appeared in my bookshelves. Not an odd occurence - Amazon frequently delivers Avoiding the Children By Hiding in the Bathroom parcels - but this one has me flummoxed, which is not something which happens often.
The Time Traveller's Wife by Audrey Niffenegger is very interesting, but I have no idea how I come to have it in my possession. I haven't bought it, nor has Mr Smat (it being a Girly Read), the Baby Smats are too little to buy GrownUp books (although Eldest Baby Smat has taken to secreting some of my books under her mountain of soft toys at the bottom of her bed - unfortunately she tends to pick the ones I'm currently reading, so I tend to notice, and as I know her hiding places.......), Mr and Mrs Smat Senior know nothing about it, and no-one else has been here recently as I have no friends and am Very Sad.
So, where did it come from?