In all fairness, I think the book should have been entitled Messiah: The Gospel According to Jessens. Charles Jessens (1720-1773), a patron of the arts and friend of Handel, also created libretti for several of Handel's choral works, including Messiah. (But that would never fly because people generally know the work by Handel's name.)
Jessens was a man of particular theological views (within the Church of England of the 18th century), and Bullard demonstrates how Jessens adapted the texts of the King James Bible and Book of Common Prayer to create a new "Gospel" (as it were).
Bullard, a professor of religion, strays off-course, perhaps, when he attempts to show how the texts might have been understood in their pre-Christian setting. As interesting as it is, it hardly contributes to an understanding of what Jessens and Handel were trying to accomplish, and if anything, such speculations tend to desacralize the work.
This reservation notwithstanding, I found this to be a wonderful addition to my permanent home library.