I have played clarinet for years, to get my grade 5 I need a certain level of music theory.
My teacher in the past has bought this book's predecessors; music theory 1 and 2. I have passed both exams with distinction so of course bought this book to start learning grade 3.
If you are teaching yourself music theory I seriously recommend getting the answer book to check yourself with and a book that walks you through the theory as this book is more questions than teaching. It also however lists for you the words that you should learn that is associated with music which tends to be a mixture of French, German and (mostly) Italian.
If you have a music teacher for your instrument then ask if they can help you and with their help you will learn quickly with this books multitude of questions on all aspects. If you have a theory teacher then I will also recommend this book to take with you to your lessons or to keep at home to test yourself. I hope this has been helpful to anyone reading this.