Sorry, misread your initial post. You want to try and secure a copy of:

The Scottish Dirk by James D. Forman, Museum Restoration Service, PO Box 70, Alexandria Bay, NY 13607-0070, USA. It has a excellent section on sgians dubh.

Forman also did an excellent article on The Evolution of the Sgian Dubh which appeared in the February, 1982 edition of "Arms Collecting Magazine".