If you struggle with punctuation, this is the book that you should acquire a copy of.
I have many, many books that I refer to when I am working. The Penguin Guide to Punctuation by R. L. Trask is probably the most accessible one on the subject of punctuation.
The definitions, explanations and examples are simple and easy to understand. The text is well laid out and not an overwhelming block. At 162 pages it isn’t an intimidating tome and you can take it with you to wherever you like to write. It is incredibly thorough (especially the chapter on commas) and even deals with aspects that are not strictly punctuation (such as capitalisation).
I recommend this book because no one should be afraid of punctuation. Sometimes it gets tricky, but most of the time it is fairly straightforward. I know that people worry about their use of commas or what a semicolon is actually for. But once you have the knowledge how, you will be surprised at your ability to wield them effectively in your writing. The guidelines are expertly set out in this book, and they will give you the confidence you need to be able to use punctuation at its best.