A really good way to learn about how musical pieces are constructed is to try and write a piece yourself. If like me you're less than fully accomplished at this, you probably need all the help you can get and if like me you've never done this before the task seems daunting, but only because it is.
This book explores the whole process from the basic motif and melodic phrase, how to join them up, vary them and set them in the right scale, mode and mood and then to develop them. It then proceeds to flesh out the process still further delving into the harmony and structure of your piece and finally on to arrangements and musical forms. Music Composition for Dummies attempts to take you on a journey from the embryonic idea you have into a more traditionally constructed and well formed piece of music. Of course, it will require no mean effort on your part, but the sense of achievement when you play your own piece is wonderful.
There are several examples of each stage of writing, but I would have liked to have seen more and would like to have seen at least one idea taken from basic motif through to completed piece. So it's not perfect and is probably not of much benefit if you are already an accomplished musician/writer. But if like me you're still learning, would like to set your knowledge of music theory in context, or fancy performing your own pieces, then Music Composition for Dummies is a great start.