I write for a living and I also train others to improve their writing and this lovely little book as been a godsend. I encourage everyone I ever teach to buy it (or get the boss to buy it for them).
It's written so well that you can relax with it over a cup of tea or read it on the train home after a tiring day's work and it won't send you to sleep! It is practical, approachable and has everything it says that good writing should: clarity, simplicity, economy, variety, vigour and suitability.
Mind you, I would have called it the Penguin Writers' Manual. I wonder if they agonised over that. Maybe they want us all to have one each, not to share.