I'd agree with anything recommended above - also any or all of Roy Palmer's excellent compilations - Abebooks has a bunch, costing from pence upwards - here:-

http://www.abebooks.co.uk/servlet/SearchResults?an=Roy+Palmer&bt.x=80&bt.y=13&sortby=3&sts=t

(Sorry, blickifier is off-line at present)

Sam Henry's Songs of the People - see the thread of that title - Q has recently added in the entire list of song-titles covered - an amazingly rich collection. Oxfam has Part One available at a rich £70, and Abebooks has a copy of the full collection for £200 (aargh!) but see the thread for more reasonable prices - I think remainder copies of the paperback version may still be available from John Moulden.

Other Penguin compilations included Australian Folk Songs and Canadian Folk Songs.

Peter Kennedy's Folk Songs of Britain & Ireland was an encyclopaedia-sized collection, transcribed from source recordings. Eaglemusicshop.com has it at £34.95 which is relatively reasonable (I think it cost £15-£20 when it came out in 1975).

You could easily overwhelm yourself (not to mention your credit-card!) if you tried to gather all of these at once. Best to start small. The first one I remember trying to learn songs from was the Clancy Brothers Songbook. References in there led me to Dominic Behan, Alan Lomax, Pete Seeger and so on. All the good books lead you on to others - you'll soon figure out which directions suit you best.

Good luck!

Ross