John Colarusso 
The Northwest Caucasian Languages (RLE Linguistics F: World Linguistics) [PDF ebook] 
A Phonological Survey

Perhaps more than any other group of languages those of the Caucasus are famous for their enormous and difficult consonantal systems. It is by no means exceptional for one of these languages to have as many as 50 consonants, and of these languages those from the Northwest Caucasus have the largest and most complex consonantal systems. The extensive use of the articulatory regions of the mouth together with the occurrence of secondary modifications at many of these points is unequalled by any other known group of languages. This detailed study examines the languages of the Northwest Caucasus and provides an essential guide to this most complicated group of languages.