Free Tune Books
O'Neill's Music of Ireland: 1,850 tunes
Francis O'Neill (1848-1936) was an Irish imigrant who became the Chicago Chief of Police in 1901. In 1903 he published this unbelievable compilation of 1,850 tunes, Many consider this “The Bible of Irish Music,”
It consists of airs, jigs, reels, hornpipes, long songs, and marches for fiddle or practically any other melody instrument.
It's free to download as a .pdf (be advise, the file is ~22 mb in size) .
Or you can purchase a bound printed copy from Mel Bay
One Hundred Songs
By James Ballantine
This is a nice collection of Aires and Songs from Scotland. In addition to music notation, it also contains words to many of the tunes.
The King Street Sessions Tune book with over 1,000 tunes This compilation is organized by tune type (airs, jigs, reels, hornpipes, waltzes, etc.) In addition to old traditional tunes, there are many contemporary tunes as well.
It's free to download as a .pdf .