Child? Hammond? Sharp & Karpeles? Jim Carroll's recordings? Each other?

In theory, yes, but in practice it seems to be mostly the professional folkies (the ones who use traditional material, anyway) that consult these sources. Your average session singer or folk club floorspotter learns their songs from the recordings made by the "professionals"

You don't usually see guest performers standing up and saying "I got this next one off a Martin Carthy/Steeleye Span/Fairport/Kate Rusby/[whoever] album..." Of course if they do a song written by someone within living memory they will credit the songwriter (if they've got any clue) but they'll probably do a quite different interpretation that makes it their own.

Anahata