If you don't own at least one of the following, I'd highly recommend them:
- Systematic Theology: An Introduction to Biblical Doctrine ( Zondervan, 1995; 1291 pages). One of the best investments you can make for the money.
- Bible Doctrine (Zondervan, 1999; 528 pages). Jeff Purswell, now of Sovereign Grace Ministries, helped to edit this volume down to over half the size. A great addition in this volume is a glossary--not including in the big ST.
- Christian Beliefs: Twenty Basics Every Christian Should Know (Zondervan, 2005; 160 pages). Wayne's son Elliot, a PCA pastor, edited this into a very simple handbook. It's probably my first choice of a basic book to get into the hands of a new believer who wants a simple guide to Christianity 101.