This book is a reference book. Reference books come in different forms. Some are long and quite in-depth, while others are more like a study guide to give direction for studying out the Scriptures for ourselves. Foster's book is in the latter category. But not only does this approach give direction and multiple biblical references for digging in more deeply in your own study, it gives us a quick reference guide to refresh our memories when studying with others. Both of these purposes are very needed and helpful.
You will find the charts and diagrams helpful and powerful in making some complex doctrines easier to understand. You will find yourself going back to sections of this material repeatedly as a quick review prior to sharing the Word with others.