G. Wilson Knight 
Mutual Flame [PDF ebook] 
On Shakespeare’s Sonnets and The Phonenix and the Turtle

First Published in 2002. This is a collection of essays and commentary on some of Shakespeare’s Sonnets looking at the areas of symbolism, time and eternity, integration and their expansion and moves onto the metaphysical poem of the Phoenix and the Turtle and considers if it has the same love as celebrated in the Sonnets.