Lina Ng’s excellent Theory of Music Made Easy is a five-book series (all available from Waltons) designed to prepare musicians for the ABRSM theory grades. Information is presented in easy bite-size sections, and each volume builds on the work of the last.
The Grade 3 book tackles more complex issues such as chord structures and compound time. There are also chapters included that act as a revision for work from earlier grades. The book is interspersed with cheerful illustrations, making learning more fun.