Written by the famous Danish linguist Otto Jespersen, this work has proved to be a valuable contribution to the English language and Linguistics. He presented theoretical considerations of language teaching in this book. As a professor of English at Copenhagen, he led a campaign for basing foreign-language teaching on the use of conversational speech rather than on the textbook study of grammar and vocabulary. He wrote several textbooks used in Denmark and other countries for English teaching. The modern techniques suggested by Jespersen make this work on didactics and pedagogy relevant even today for English teaching.
I am always amazed when books, especially books on didactics and pedagogy, written over a hundred years ago can still be used today. Of course, some tweaking is necessary, but I can see several things I can apply to teaching English right now.