Another great little book is "Tartan: the Highland Habit" by Hugh Cheape (ISBN# 0 948636 70 X).

I agree that the Osprey books are af great value, especially for the period images and modern renderings (regardless of one's opinion of Mr Reid!).

My reenactment friend Mara Riley is extremely knowledgeable on the subject as well. Here is her website:

http://www.marariley.net/celtic/scotland.htm