The Vestiarium Scoticum (full title, Vestiarium Scoticum: from the Manuscript formerly in the Library of the Scots College at Douay. With an Introduction and Notes, by John Sobieski Stuart) was a book which was first published in 1842 by William Tait of Edinburgh in a limited edition. John Telfer Dunbar, in his seminal work History of Highland Dress, referred to it as "probably the most controversial costume book ever written".
The book itself is purported to be a reproduction, with colour illustrations, of an ancient manuscript on the clan tartans of Scottish families. Shortly after its publication it was denounced as a forgery, and the "Stuart" brothers who brought it forth were also denounced as impostors for claiming to be the grandsons of Bonnie Prince Charlie. It is generally accepted today that neither the brothers themselves nor the Vestiarium are what they were purported to be.
Nevertheless, the role of the book in the history of Scottish tartans is immense, with many of the designs and patterns contained therein passing into the realm of "official" clan tartans.
More information on the Wikipedia page