Interesting for historical value, but a poor choice for someone learning the art of composition today. While it is unlikely that anyone following the advice given in this work will be marked down for grammatical errors, they may get marked down for being too dry or wooden if they were to follow Mr. Devlin's advice too closely.
His persistant use of "viz." would be considered archaic by most American readers, as would his inclusion of "shall" as a helping verb.
I have tried unsuccessfully to determine when this book was written, but it reads as though it were written about 50 or 70 years ago, which is why I found out fascinating from an historical point of view.