Jenkins notes on p. 179 that “there is no compelling linguistic reason for the comparative study of Greek and Latin.” Nevertheless:
The 20th century standard was Buck’s Comparative Grammar of Greek and Latin (Main Library 3rd Floor PA111 .B922C – currently checked out!); Jenkins (no. 528) notes it is “now badly dated” (it was originally published in 1933.)
The new standard is Sihler’s New Comparative Grammar of Greek and Latin (1995), (Main Library 3rd floor PA111 .S54 1995 – also checked out, yay!) Jenkins (no. 540) describes it as “valuable” and notes in detail the differences between this work and Buck – of which this was originally intended to be a revision.
Searches of UGA’s GIL catalog for the subjects “Latin language – Grammar – Comparative Greek” and “Greek language – Grammar – Comparative Latin” have, as one would hope, a nearly 100% overlap.
I also want to mention a new work, Michael Weiss’ Outline of the Historical and Comparative Grammar of Latin (2009), which was recently reviewed by James Clackson at BMCR. Clackson describes it as “by far the most comprehensive and reliable compendium of the historical and comparative grammar of Latin available in English, and even gives the monumental work of Leumann (1977) [Lateinische Laut- und Formenlehre (Man Library 3rd Floor PA25 .H24 Ser. 2, Sect. 2, v. 1, rev. 1977)] a close run for its money in terms of scope and coverage.”
Becoming a Classics Librarian 