Indo-European Origins of the Celtic Verb

Click on the link for this seminal work by Calvert Watkins (1969) that sought to explain the origin of the Celtic s-subjunctive:

Watkins, Indo-European Origins of the Celtic Verb

For later scholarship, see especially The Brittonic Subjunctive and Future, colloquium paper by Jay H. Jasanoff (1993): competing ideas (Rix, McCone) are discussed, but Jasanoff concludes that the s-subjunctive is indeed ultimately an outgrowth of the Proto-Indo European s-aorist.

Jasanoff, The Brittonic Subjunctive and Future