John Purser
Scotland's Music
Mainstream, London, 1992 & 2007.
Originally published in 1992, this large hardback is a great compilation of information and opinion about all aspects of Scottish music, with many illustrations and a great number of music examples transcribed into staff notation - most of them otherwise unpublished and unavailable elsewhere. The content is not entirely reliable but the book is very useful for discovering the wider context.
A much enlarged second edition was published in 2007, incorporating a lot of new material. Both editions are now out of print and very hard to find.
To accompany the first edition there was a companion double CD. However this is now out of print (though I often have a secondhand copy available).

See also John Purser's edition of the Caledonian Pocket Companion