In "A Preface To Paradise Lost", Lewis talks about what Milton was trying to accomplish with this epic, and this critical work will always stand in very high circles indeed in Milton scholarship. Much of his wisdom is sound, especially when it comes to judging a thing. Lewis says to judge something we must know what it is and what it is intended to do, and then he goes into types of epic poetry and what Milton is trying to accomplish with his work.
Milton's influence on "Perelandra" is very evident, for Lewis took everything he did not like in Milton and threw them out in the elaborate construction of "Perelandra". It is obvious Lewis greatly admires Milton, and his tribute to him is very great indeed.
One issue, as some critics have pointed out, is Lewis's inability to appreciate spoken poetry, which is a very real weakness. Other than that, however, "A Preface to Paradise" lost stands as one of Lewis's best work of scholarship and will greatly add to anyone's understanding of Milton's aim and theology in general.
Lewis deals with the several key issues in Milton. One thing often misunderstood by the Romantics was it seemed to have a positive portrayal of Satan, and how lordly he sounds. The idea of better to be a king in hell than a slave in heaven is of course an absurd one, but from this Lewis builds his great little novel "The Great Divorce."
Ultimately, like Tolkien with "Beowulf: The Monsters and the Critics", Lewis has written the definitive textual commentary to "Paradise Lost", and probably the most read book on Milton as well as far as literary criticism goes.
[Throughout the years, I have written a number of reviews that have never been published online on Amazon. These writings comprise two types of reviews: unfinished reviews, abandoned during various stages of composition, and completed reviews that for life reasons were never posted. Of the later type, back in September 2001 I wrote a cache of work, a full sixteen reviews of several different C. S. Lewis books which have never been released. I am publishing these reviews now for the first time, over a decade after they were initially written. Mike London 10-3-2012]
*(These reviews covered all seven books of "The Chronicles of Narnia", the three novels of "The Space Trilogy", "The Abolition of Man", "The Four Loves", "A Preface to Paradise Lost", a revised version of my 2000 review of "Till We Have Faces", "Surprised By Joy", and "The Screwtape Letters".)