Editor of the major anthology 'The Mabinogi: a book of essays' (NY: Garland Publications, 1996)
Republished (NY: Routledge 2015).

The 1970s - 90s was a pivotal period in Mabinogi Studies, a paradigm shift, and Sullivan's anthology neatly captures the shift in its collected essays from the period. The 2015 reprint sold out within only two years, demonstrating how important it is to any serious student of the Mabinogi. (Some of its content is available on googlebooks, and of course via libraries.)
Sullivan explains that he just got tired of tracking these key articles in so many different places. So he put them in a book. His Introduction is genuinely helpful as an overview of Mabinogi Studies up to 1996.
The student needs to be aware that much of the book is packed reading, which cannot be absorbed in one read through. But patiently re-reading each article, will repay the effort. See notes below the Contents list for help.

Sullivan, Charles William, ‘Preface’, pp. xi-xiii.
Sullivan, Charles William, ‘Introduction’, pp. xv-xxi.

# ORIGINS
Bromwich, Rachel, ‘The Mabinogion and Lady Charlotte Guest’, pp. 3 – 18.
Charles-Edwards, T.M., ‘The Date of the Four Branches of the Mabinogi’, pp. 19 -58.
# COMPARATIVE ANALYSES
Wood, Juliette, ‘The Calumniated Wife in Medieval Welsh Literature’, pp. 61 -78.
Keefer, Sarah Larratt, ‘The Lost Tale of Dylan in the Fourth Branch of the Mabinogi’, pp. 79 -98.
Ford, Patrick K., ‘Branwen: a study of the Celtic affinities’, pp. 99 -120.
Welsh, Andrew, ‘Manawydan fab Llyr: Wales, England, and the “New Man’, pp. 121-141.
# STRUCTURE
O'Coiledin, Sean, ‘A Thematic Study of the Tale Pwyll Pendeuic Dyuet’, pp. 145 -151.
Hanson-Smith, Elizabeth, ‘Pwyll Prince of Dyfed': the narrative structure’, pp. 153-164.
Bollard, John K., ‘The Structure of the Four Branches of the Mabinogi’, pp. 165 -196.
Ford, Patrick K., ‘Prolegomena to a Reading of the Mabinogi: Pwyll and Manawydan’, pp. 197 -216.
Jones, R.M., ‘Narrative Structure in Medieval Welsh Prose Tales’, pp. 217 -262.
# THEMATIC
Gantz, jeffrey, ‘Thematic Structure of the Four Branches of the Mabinogi’, pp. 265-275.
Bollard, John K., ‘The Role of Myth and Tradition in the Four Branches of the Mabinogi’, pp. 277 -302.
McKenna, Catherine, ‘The Theme of Sovereignty in Pwyll, pp. 303 -330.
Valente, Roberta, ‘Gwydion and Aranrhod: Crossing the Borders of Gender’, pp. 331 -345.
Sullivan, Charles William, ‘Inheritance and Lordship in the Math’, pp. 347-366.

Works Cited, pp. 367-382.
About the Contributors, pp. 383-387.