OK, back to topic,
Here is a source for some of the books mentioned:
http://giftshop.scottishtartans.org/books.htm
That is the museum (X Marks sponsor) that Matt Newsome curates.

Ex:
Old Irish & Highland Dress by H. F. McClintock
Originally published 1943. This classic and elusive work provides a detailed study of the dress of the Highlanders and the Irish in ancient times, with illustrations drawn from archival sources. It provides a wealth of information on the dress of these two closely related Celtic lands. Many of the illustrations are unique and are invaluable in understanding the development of men's and women's dress in the period from earliest times through the eighteenth century. There is a chapter on the dress of the Isle of Man as well.
Available on CD-ROM only (Adobe software included) for $12.95

Sometimes, there are other good books at the museum gift shop that are not in the on-line gift shop, such as the aforementioned Osprey series. Matt will know what is a good one for your purpose out of what they have.