Mudcat's a good place to start but there are some alarming omissions; better still is to dig into the threads themselves, where one finds songs not included & some highly informed discussion on variants. The Penguin Book has been touched by the hand of Bert Lloyd and may not be all it pretends to be, but still an invaluable reference book. I would have though such a book would be a permanent fixture on the bookshelves of anyone with even a passing interest in traditional song. The songs are given with single-line melody, with a few examples of suggested accompaniment, but being musically illiterate (not out of choice, it's actually a form of dyslexia) this doesn't bother me in the slightest, tending as I do to source songs from singers & recordings, and effect my own accompaniments accordingly*.

I believe there is a new edition, with EC on the cover, rather than the old dancing bear, but otherwise it's the same book. The deeper you dig the better it is, generally speaking. Scour the second-hand bookshops & flee-markets - there's some good book stalls at Tynemouth Station on a Saturday where I've picked up any amount of old ballad collections and stuff; Bob Copper's A Song for All Seasons for example and the invaluable Faber Book of Popular Verse.

* Interesting perhaps in the context of this thread, is that whilst I do use a variety of instruments to accompany my performances, I rarely deviate from the melody of the song I'm singing, nor ever use chords as such.