There are a number of references and books that I have encountered and not been able to find:

Reflections on the rise and fall of the ancient republics, adapted to the present state of Great Britain, Edward Montagu. Published by A. Millar, London, 1759. Supposedly reprinted `in numerous editions'.

Translated into German in Brenslaw (?) in 1769.

Also printed as:
Riflessioni sopra la elevazione e la tendenza delle antiche repubbliche adattante al presente stato della Gran Bretagna, Odoardo Montagu. Published in Undine(?) by Gallici(?) in 1781.

Also printed as:
De la naissance et de la chute des anciennes republiques, translated by Cantwel(?). Published in Paris, 1793.

The Career, Lands and Family of William Montague, Earl of Salisbury, 1301-44, Robert Douch, PhD dissertation.

http://www.montaguemillennium.com/familyresearch/books.htm