What Is Reformed Theology? Understanding the Basics by R. C. Sproul is an introductory book on Reformed theology. It was originally published in 1997 and has sold over 100,000 copies. The book talks about the foundation of Reformed thought, as well as the five points of Reformed theology – which are Humanity’s Radical Corruption, God’s Sovereign Choice, Christ’s Purposeful Atonement, The Spirit’s Effective Call, and God’s Preservation of Saints. A nice thing about the book is that it focuses on Reformed theology as a whole instead of specifically discussing Calvinism.
The book is well organized, and it is a valuable resource for both Reformed Christians as well as those who are not in the Reformed camp. Because Sproul is a Calvinist, the book is obviously going to be in favor of Reformed thought; readers who would like to see an opinion from the Arminianism side might check out Against Calvinism by Roger E. Olson.
You can find What Is Reformed Theology? here.