Protestants and Catholics are not as separated theologically as they may think.

In Roman Catholics and Evangelicals, Norman L. Geisler examines the common spiritual roots between Catholics and Protestants and the theological and moral bridges upon which they can both travel. He explores similarities and differences in both doctrine and practice. He also speaks to interesting relationships and alliances that have developed between Catholics and Protestants.

This book is an excellent reference tool and textbook.