Passbooks: - National Learning Corporation has been a leading publisher of test-preparation Passbooks(R) for more than 50 years, with 6,000 titles available across 22 series. The College Level Examination Program (CLEP) enables students to demonstrate college-level achievement and earn college credit in various subject areas based on knowledge acquired through self-study, high school and adult courses, or through professional means. Each book in the CLEP Series offers practice through multiple-choice questions and answers in relevant subject areas, as well as informative texts to aid in understanding the study material.