Once upon a time, in 1781, an Edinburgh antiquarian published a book proposal. He never published the book. There's apparently only one copy of the proposal in the whole world, and it's in the National Library of Scotland.
Well, it is actually missing from where it should be, in the National Library of Scotland. I've just been to Edinburgh - had a great day, read other useful stuff - but didn't see the one, unique thing I set out to see.
Who is the patron saint of libraries? Or of lost causes? Because I really, truly do need that book proposal to be found! Urgently!