There’s a reason seminaries and Bible colleges around the world use Wayne Grudem’s Systematic Theology. Grudem’s work is a rare combination of evangelical scholarship, accessible theology, and practical application. The size of the book will intimidate some, but most readers will be able to work through this book. Even if you don’t read Systematic Theology cover-to-cover, it’s a great resource to have on hand for quick reference.
Additionally, for those who want the “Cliff’s Notes” version of Systematic Theology should check out Christian Beliefs: Twenty Basics Every Christian Should Know. This is a great place to start for pre-teens and teenagers who want to learn more about the Bible and theology.