Valery, One of the best sources of information on Scottish songs is the five volume set "The English and Scottish Popular Ballads" edited by Frances Child. It's available in paperback and used to be published by Dover Publications Inc, 180 Varick Street, New York, N.Y. 10014.For more Scottish stuff, try "Ord's Bothy Songs and Ballads" published by John Donald Publishers Ltd, 138 St Stephen Street, Edinburgh, EH3 5AA Scotland.
The Greig-Duncan Collection is also currently being brought out in several volumes - I'm not sure who is publishing it though.
Anne