Victor Durkacz (1983) The decline of the Celtic languages. Edinburgh UP.

People may also be interested in:

Ken George (1986) The pronunciation and spelling of revived Cornish.
                   Cornish Language Board [which does not print its
                   address on its publications!]

A brief traditional [Priscian-] style morphologically-heavy grammar is:

R. Morton Nance (c. 1958) Cornish for all. Federation of Old Cornwall Socs.

To my mind by far the best book on the history of Cornish vocabulary,
and to some extent its grammar also, is:

Oliver Padel (1985) Cornish place-name elements. English Place-Name
                   Society.

Though obviously its coverage is somewhat limited, the limitation is
nothing like as severe as one might expect in such a book. And the
philology is of the highest order.


Richard Coates
University of Sussex