This book was bought when a friend of mine had to write an essay about Tolkien's works.
Apart from insights about the creation of the Elvish languages, references to his own books are scarce, however, and people who are interested solely in 'Middle Earth' will probably be disappointed - or else surprised, as I was, to find that Tolkiens thoughts on (more or less) 'academic' subjects can be just as enchanting as his invented stories.
I started to read the book out of a mild interest in the author, as I have loved the 'Lord of the Rings' and 'The Hobbit' since I read them as a child, and was astonished at how much I enjoyed some of his essays.
This edition contains:
Beowulf: The Monsters and the Critics On Translating Beowulf Sir Gawain and the Green Knight On Fairy-Stories English and Welsh A Secret Vice Valedictory Address
As I'm not familiar with Beowulf, I must admit I skipped those essays. I didn't know Sir Gawain and the Green Knight either, but the full text can be found in the web, and after reading the story I enjoyed Tolkiens take on it very much.
'On Fairy-Stories' and 'A Secret Vice' (about his passion for inventing languages) are the essays I enjoyed most. The book is definitely more than worth buying, just for those two. You don't need any special prior knowledge to appreciate them, though 'A Secret Vice' touches on linguistics. Tolkiens style is incomparable. He has the gift of expressing complex thoughts in an endearingly simple, and yet beautiful, way. He lures you (mostly along meandering bypaths :) into the world of one of the most imaginative authors ever, and allows you to catch a glimpse of his fascinating personality.