According to the author (in this preface) the book is

  this book consist of lectures given by me at Cambridge. Though they have been largely rewritten, I have kept a good deal of their original lecture-form, as being (I hope) rather less formal and less dogmatic. For to dogmatism, those who write on language seem, for some reason, particularly prone; and I should like to make clear at once that, if at times I have put my view strongly, I do not forget that such matters of taste must remain mere matters of opinion.
In addition I have included a good many specimen passages from various authors. Perhaps I have quoted too much. But a book on style without abundant examples seems to me as ineffectual as a book on art, or biology without abundant illustrations. Many of these passages are in French. That may be Gallomanian on my part and I must apologize if they trouble some readers. But some ability to read French prose does seem to me most desirable for anyone who would write well in English. I have tried to choose pieces not too difficult in syntax or vocabulary. And in these days less than ever can we afford to be better insular.