As a person whose life is dedicated to art in all its various incarnations, this has proved the single most enlightening work I have ever had the pleasure of reading. While it itself is literary criticism and in one sense not literature but a study thereof, it's the most radical, revolutionary book I have read regarding art. Before I can continue, one point needs to be cleared first.
I'm a Christian, and I believe the single most important priority is to lead people to the knowledge and saving grace of the Lord Jesus Christ. You can do such through art. However, anything that Lewis wrote that lead someone to Jesus is, of course, more important than this book in that respect. Jesus comes first, art comes underneath that in priority, as do all things. That being said:
AN EXPERIMENT IN CRITICISM is the single most important work C. S. Lewis has produced when it comes to literature and the arts. THE CHRONICLES OF NARNIA, the SPACE TRILOGY, and TILL WE HAVE FACES are literature, but this overwhelms them all - not because of what it is (a universal principle that can be applied to art), but because of what it is not (a story or work of art that not everyone will have the same taste for). People may or may not like his fiction (although I find it rare to meet a person who doesn't like NARNIA) - but this book anyone can appreciate, especially those interested in literature in specific and art in general (for, although it concerns itself primarily with literature, this book also stands in defense of drama, music, painting, and the artistic endeavours of humankind in general). Because there are differing tastes in terms of fiction, people who will not read Lewis's own literature will (or should) read this. This element comes into play at the last chapter, where Lewis brings out how hard it is to take down a work with this apparatus, because, while you may not enjoy a work, others may. Literature is a very highly subjective experience.
CRITICISM's central argument rests in the fact that books should not be judged by some arbitrary critical analyses, but by what response it elicits in the reader. This book contains one of Lewis's famous quotes, at the end of the Epilogue: "But in reading literature I become a thousand men and yet remain myself." That is the central thesis of this work: not to approach a piece of literature, or movie, or painting, or drama, as something to experience and forget immediately afterward, but to enter into it, surrender to it, and approach it with an open mind. That is one thing that is so great about this book - even people who have prejudices against Lewis can read this work.
It also points out the central flaw in evaluative criticism: it may dominate and wipe away the chance at a reader appreciated the work for what it really is, and to experience it in the reader's own way. Lewis does not argue that all evaluative criticism is bad - there's a very human need, he says, to `compare notes', and that is normal, but to much has been given over to this `note comparison' and not enough to the actual work of art. People, especially students (Lewis was, by profession, a medieval literature professor), had quite a broad range of knowledge concerning Chaucerian and Shakespearian criticism and hardly any of Chaucer or Shakespeare (he cited this example at the end of the last chapter). To much has been given over to criticism.
Although I will continue to write reviews, this book has forever changed my approach. Lewis states that one can have an appreciation of a work without the critics, but one CANNOT have an appreciation of the critics without the author. Now I propose that all reviewers should read this book, and keep this in mind when writing. I certainly will. It is also my personal belief that anyone in universities who are studying literature should be required to read this book at the start of their very first semester, so they may examine their motives of precisely WHY they are in this study.
Indeed, the biggest tragedy of this book is, I fear, it is not highly enough read. Regardless of your views on C. S. Lewis, this is one book everyone should read who professes a love for art, and ESPECIALLY by all who write literary criticism. And while that profession does have a place in our world (where would academia be without it? `Publish or perish!'), it is superseded in importance by the art that it deals with, and we should first immerse ourselves in it, sometimes several times over, before we turn away and reach for that scholarly volume. And if it's a good work, it will only encourage you more to go to the work at hand and discover for yourself what the art can do for you. (Shippey's ROAD TO MIDDLE-EARTH is a good example of a well-written criticism). Lewis said one good element about criticism is, if the critics truly care for the art, the enthusiasm will be apparent, and it may cause you to read literature otherwise unknown to you. Also, a central element is the difficulty in producing condemnation to a work. It's a good case against censorship, because although a great work can be abused, it can also be used properly. One may classical images and use it as pornography - while one may look at it and fall in love with the Renaissance. Of course, PLAYBOY is mainly used for lust.
A side note: Pay close attention to the chapter on MYTH, which is a central element in both Tolkien and Lewis. This alone should make it required reading for anyone who study the lives of these two great Christian writers. This work also shows you the depth of versatility of how well-read C. S. Lewis actually was, and shows his phenomenal memory of such things.