• B. F. C. Atkinson, The Greek Language 1931The following were listed at various times as being in preparation, but as far as I can tell were never completed:
• T. Burrow, The Sanskrit Language 1955
• W. D. Elcock, The Romance Languages 1950
• W. J. Entwistle, The Spanish Language, together with Portuguese, Catalan and Basque 1936; second edition 1962
• W. J. Entwistle and W. A. Morison, Russian and the Slavonic Languages 1949
• A. Ewert, The French Language 1933
• R. A. D. Forrest, The Chinese Language 1948
• Einar Haugen, The Scandinavian Languages
• Bruno Migliorini, The Italian Language (“abridged and re-cast by T. G. Griffith”) 1966, 1984
• L. R. Palmer, The Latin Language 1946
• R. Priebsch and W. E. Collinson, The German Language 1934
• R. A. Crossland, The Anatolian LanguagesIn two cases (English and Scandinavian), different prospective authors are given in successive versions of the list of forthcoming volumes. Neither N. Davis nor Helge Kökeritz apparently ever completed The English Language, but Einer Haugen's Scandinavian Languages did appear, replacing Sommerfelt's. The focus of Kenneth Jackson's volume was apparently shifted, since he is assigned distinct but related topics in different versions of the list, but in any case I can find no record that his contribution was issued. Unpublished titles were still being listed as forthcoming on reprints as late as 1968, presumably because Faber did not go to the trouble of changing the plates.
• G. Bonfante, Indo-European Languages
• G. R. Driver, The Hebrew Language
• Kenneth Jackson, The Celtic Languages
• Kenneth Jackson, The Gaelic Languages
• N. Davis, The English Language
• Helge Kökeritz, The English Language
• Alf Sommerfelt, The Scandinavian Languages
Of the volumes I've seen, Elcock's Romance volume is perhaps the most easy to recommend to a non-specialist like myself; the Spanish volume is also of interest to any devoted student of the language. All of the volumes have a certain amount of abstruse philological jargon in spots, but the historical sections and examples are accessible for anyone who's interests run to this kind of thing. Forrest's Chinese Language, however, is pretty forbidding for anyone without a linguistics background.