My favorite essays on Wilbur are Alan's (Sewanee Review and Chronicles, which can be read here:
http://poetry.seablogger.com/), and the major paper Rhina Espaillat delivered at West Chester. David Alan Jeffrey does not have the poet's advantage that Alan or Rhina has. However, he knows his Bible cold. As well as I know Wilbur's work, I gained a great many insights into poems I thought I knew inside out.