The Scottish Regiments Second Edition
This book is not specifically related to the First World War but it is in the Collection because it is a description of the state of the Scottish Regiments as at the date of its publication (1996), that is, before the creation of the Royal Regiment of Scotland.
This means that the state of the regiments is a little different to what it was in the war, but those regiments which were standalone at the time of the war and had since been absorbed, amalgamated, whatever, are easily identifiable.
Otherwise the book is a valuable guide to the history and also the recruiting areas of the different regiments.