This book is incredibly useful for anyone writing prose, be that fiction or non-fiction. It begins with several recommendations concerning the use of words, grammar, and punctuation. It moves on to a list of words that are often misused (or overused, in the author's opinion); and the final chapter is on how to develop your own writing style.
First written several decades ago, it still has a refreshing directness to it ("'Prestigious' is an adjective of last resort: it is in the dictionary, but you don't have to use it"), and, though some of the advice is the opinion of the author only as opposed to reflecting concrete grammatical rules, I couldn't help but agree with nearly everything that he pointed out; for example, his advice that words such as 'very', 'rather', 'pretty' and so on should be avoided if possible.
Interesting reading, though perhaps more of a reference book. The introduction and final chapter are good on how to apply the principles talked of in the body of the book itself.