Understanding the Lord of the Rings: The Best of Tolkien Criticism contains a good collection of Tolkien criticism. The only thing that seems to be missing are essays dismissing the literary merits of Tolkien's fiction. I expect that to offer interesting insights into his work and to leave it out renders the collection somewhat unbalanced.
Note that the book reprints several, but not all, essays from Tolkien and the Critics: Essays on J.R.R. Tolkien's The Lord of the Rings (by the same authors).