Hi Rikard - A lot of 'lore' is passed on from player to player (and can change as it goes!) there isn't any one single resource with the background on the tunes. Some specific stories can found in different non-NPS books - or on sleeve notes from recordings.
The A5 format tunebooks are very much designed and published as straight music books - maximising the quantity of music for the player. The 3rd NPS tunebook does have some notes on the tunes at the back but they are quite basic; Green and Yellow pocket books also has some snippets. The Clough and the Billy Pigg books are much larger format and have more context as they are focused on a specific repetoire and playing style.
Thinking about it, you're perfectly correct ... I could write a book on this ... but I don't think there's sufficent demand! Just thinking about the 1st NPS book and picking a few tunes at random ...
- Chevy Chase, very old tune named for the ballad, appeared in the Northumbrian Minstrelsy but not in Peacock ... original melody was simpler and called 'Flying Fame' and that melody was used for songs in Shakespeare's plays
- Sir Sidney Smith's Sir Sidney was very famous in his time (1764 - 1840), amongst other things he served in the Swedish Royal Navy and is known as the Swedish Knight. The tune itself has an odd length - but in can be found in older manuscripts without the extra four bars. Clough family used to play this as a showpiece duet - each playing working round the house in opposite direcitons and meeting back at the front door in perfect time ..
- Whinshield's Hornpipe was written by J L Dunk - who was a bit of an oddball and into experimental music in the 1930's. The tune as printed bares only a passing similarity to the original manuscript - someone involved in the 1st tune book did some fairly heavy editing to get the (excellent) tune we have today
- Hesleyside Reel & Bellingham Fair were both written by T J Elliot. 'Tommy Elliot' was a fiddler and by all accounts a piano tuner living in and around Gunnerton in the early 20th century. All of his tunes (5 or 6) reference places in the North Tyne valley (lower half).
- Jacky Layton this old tune appears in many many different sources and locations. Jacky himself is reputed to have been a champion dancer who accepted a challenge to dance from one town to the next, succeeded and died shortly afterwards from his exertions. The towns and distance vary depending on the source - but it's usually stated as being in Ireland, 8 or 20 miles and Morristown and Dublin often feature.
Looking at the index - I've anecdotes on over 80% of the tunes in that one alone ...
Best thing you can do is play some of the tunes, fall in love with one or two and then ask if there's any story.