If you are ever in a pickle over your grammar, then I really recommend The Elements of Style. It is a short, quick to read, easy to follow, and clears up all those little questions that nag at you when writing.
The guide was originally written by the American professor, William Strunk Jr., who taught English at Cornell University. In 1918, he self-published a forty-three paged booklet for his students that advised on the rules of the English language, and the most commonly made mistakes.
One of his students was E.B. White, who is most well known as the children's author of Charlotte Web. White was commissioned by Macmillan Publishers to create a revised copy in 1959. Since then, there have been three further editions, the most recent of which was published in 2000. This is the edition I own, and it is just under one hundred pages.
I bought the book because it was on my reading list when I was at university, studying Creative Writing, and found it extremely helpful. Although it is American, it is just as useful for British English, or any other variation of the language. That is because the book is about composition, not spelling. It helps with syntax, grammar, and they way things are phrased.
Chapter one covers the basics, such as where to place commas and semi-colons, and also explains the difference between simple, compound, and complex sentences. In the next chapter you will learn how to make your sentences more clearer and more engaging. For example, Strunk & White explain the importance of using concrete nouns, active voice, and positive form.
There are five chapters in all, but I find chapter four the most helpful, as it deals with words and expressions that are commonly misused. Strunk & White taught me that 'all right' is two words, not 'alright', one. Strunk & White taught me the difference between 'less', which refers to quantity, and 'fewer', which refers to number.
Considering when The Elements of Style was first written, there are a few rules in the book that are a bit outdated. For example, Strunk & White still instruct readers not to use a split infinitive, - putting an adverb in between 'to' and the following verb - but that has been out of style since Captain James T. Kirk said 'to boldly go where no man has gone before.' I have read many other grammar books that state this puritan rule is silly.
Apart from the odd obsolete law it imposes, The Elements of Style is an essential 'how to' and 'how not to' on writing.