To pg. 12 fn. 7 add to the general bibliography for Celtic Ball and Muller 2009 which refers to the second edition of Ball, Martin and Nicole Muller (eds.), The Celtic Languages. Routledge: New York. This edition is a significant improvement over the first in that it has essays covering the Medieval stages of the Celtic languages by David Stifter (Early Irish) and David Willis (Old and Middle Welsh). Joe Eska discusses "The Emergence of the Celtic Languages" and (together with D. Ellis Evans) "Continental Celtic."
Another second edition which should be added is Horrocks 2010 (Geoffrey C. Horrocks, Greek: A History of the Language and Its Speakers. Chichester/Malden, MA: Wiley-Blackwell) replacing Horrocks 1997 on pg. 18, fn. 53. See the review at BMCR.