Wikipedia: Advanced Placement Exams
Helpful Wikipedia overview of the AP Program, its history and significance, with links to articles about each test, past tests no longer offered, and more.

College Board: AP Central
The official site for College Board Advanced Placement Program. Contains information covering all aspects of the AP's. See also: AP Students.

What is AP?
General information about the AP Program, what it takes, the rewards, how to enroll, AP Awards, etc. AP Exams Overview provides additional details.

AP Courses and Exams
An overview of each course plus info on related majors and careers, resources, and course goals. See also: Course and Exam Pages and Exam Timing and Instructions.

Calendar for AP Students and Exam Dates
Exam dates/times for each AP test, plus pre-course planning tasks to complete in the months leading up to the start of class and info on exam fees and fee reductions.

About AP Scores
Information on scoring, average scores submitted to colleges, AP Awards & Recognitions, score access schedule, sending scores to colleges, and more. See FAQ here.

Princeton Review AP Test Prep Books
PR books are generally strong and take a sound "content plus strategy" approach to test prep. They're a good place to start any prep program, including AP prep.