This is a generalisation, but in terms of repertoire I find I get more out of songbooks than I do recordings or live performances.

But, to again massively generalise, that's partly because I'm more interested in the words than the tune. And often a recording you'll be hearing a performance you might not like, whereas looking at words on a page, or even a tune on a page, you get more of a blank slate to work with.

I realise that's rather against the 'oral tradition' spirit of folksong. But that's just how it is for me. (Plus I often find that if I've just heard a fantastic performance of a fantastic song, well, the last thing I want to do is learn it myself. I want to proselytise about how wonderful the singer is, and how everyone ought to buy their album, or go see them live.)

Anyway, this year I've been immersing myself in these books, among other things:

The Penguin Book of English Folk Songs - Ralph Vaughn Williams & Bert Lloyd

Folksongs & Ballads of Scotland - Ewan MacColl

English Country Songbook - Roy Palmer

you can get all the above via amazon

but you could do worse than just check out your local library, and get out, or order, some songbooks. I'm in Brixton, and Lambeth libraries have a surprisingly large stock of songbooks in their collection (Cecil Sharp, Francis Child, Lloyd and others)

PS
Just cos loads of people have famously sung a song, doesn't necessarily mean anyone's ever sung it well