What seems to happen is that something I've come across in a pub or session or concert very often turns out have a source in a book or a recording.

The couple of examples from the last week or so are the Beggar's Song, which I've heard live a few times, finding it later on a Belshazzars Feast CD and probably from Gammar Gurton's Needle via Steeleye; and Alfred Noyes The Highwayman. A woman sang this very well - dramatic without being melodramatic - yesterday, I knew I'd heard mention of it recently and in today's Guardian paper is mention of the late Don Partridge having sung an arrangement of it.

There is stuff I only ever hear in particular places though.