Beowulf: The Monsters and the Critics can be called the most important article on Beowulf of the 20th century. Incredible as it may now seem, prior to Tolkien, Beowulf had been seen primarily as a curious linguistic-literary artifact, useful as a source of information about the early Germanic past (customs, language, laws, toponymy, etc.).
Tolkien was the first critic to draw attention to the poem as a poem and to point out that the central literary structure of the tale revolves around the hero's battles with them monsters, which previous critics had dismissed as mere fabulous emendations to a tale whose primary value was historical.
Originally published by Oxford University Press in 1937 (2nd ed. 1958, 3rd ed. 1969) and by Folcort Press in 1969, Norwood Editions in 1975, R.West in 1977 and Arden Library in 1978.
The essay is included in a lot of books, f.e. in The Monsters and the Critics and Other Essays.