Chester E. “Checker” Finn Jr., the nation’s most talented writer on U.S. education policy, has just produced the most comprehensive book on Advanced Placement, the most powerful educational tool in the country.

That’s too many superlatives for one sentence. I will lose points on the test. But having long been intrigued with the influence of AP’s college-level courses and exams, I was thrilled to see Finn and his co-author, Andrew E. Scanlan, add so much to a subject crucial to the future of high schools.

Finn learned about AP much earlier than I did. In 1962, I was a 12th-grader in a California public high school with no AP courses. That same year, Finn was admitted to Harvard University with sophomore standing because of his success in several AP courses at the Phillips Exeter Academy in New Hampshire.

Finn is distinguished senior fellow and president emeritus at the Thomas B. Fordham Institute in Washington, and Scanlan is its research and policy associate. Their new book is “Learning in the Fast Lane: The Past, Present and Future of Advanced Placement.” It presents the good and the bad of AP’s growing grip on high school curriculums. It explains with lively wit how homework-hating teenagers have adjusted. It looks at AP under different conditions in Texas, New York, Ohio, Maryland and several big charter networks.

About 3 million AP students sit each year for 5 million tough exams, graded independently by professors and veteran teachers who don’t know and don’t care whether you were nice to your teacher in class. The 64-year-old program, the book said, “has quietly worked its way into the offerings of most public and private schools, the policies of many states and districts, the admissions and placement decisions of hundreds of universities, the educational aspirations of countless families, and the academic programs of innumerable college students.”

Finn and Scanlan said AP is “a nearly unique standard of rigor and quality for the K-12 system, a source of professional qualification for myriad teachers and — remarkable in these fractured and politicized times — a de facto national high school curriculum joined to a battery of exacting tests that are widely deemed ‘worth teaching to.’ ” Because it is not a government program, the authors noted, it has escaped the damage done to political projects such as the Common Core State Standards.

There are still difficulties. AP has competition from dual credit, also called dual enrollment or concurrent enrollment. Those are local college courses for high school students. They offer guaranteed credit for adequate work without requiring a terrifying three-hour exam. A few of the most selective colleges refuse to give credit for AP or do so only for top grades on the exam. Exam fees and teacher training costs can mount.

The nonprofit College Board runs the program. You think its SAT and PSAT tests are a bigger deal? Finn and Scanlan say no: “We estimate that fees from AP tests and instructional materials make up about 47 percent of the College Board’s total revenue (which means they brought in some $446 million in 2016), more than it now receives from the rest of its vast catalog of assessments, including the high-visibility SAT.”

What about the seven prestigious Washington-area private schools that announced they are dropping AP? They are a rounding error compared with the surge of private schools into AP, from 3,085 in 2006 to 5,179 in 2018. The defection of the “Washington Area Seven,” the authors said, “is a mouse alongside the fattening hippo of schools” adding AP.

Finn and Scanlan discovered one factor I have not found in any stories about AP, including mine. They said some private school heads have confessed that “the rigorous marking of AP exams actually helps them and their teachers push back against grade inflation and withstand pressure from aggressive parents to elevate their children’s report cards.”

The College Board has persuaded many state legislatures to require state university credit for scores of at least 3 on the 5-point AP test. That makes the program vulnerable to political winds. What happens if some professors object and legislators decide it’s unfair to insist on credit for 3s, and revoke that rule? If students cannot rely on credit for a 3 or even a 4, they might turn instead to what the authors call “the cheaper, easier, and more assured path of dual credit.”

Having read this fine book, a student could write an insightful essay on the program’s strengths and weaknesses. What of AP’s future? That’s beyond the purposes of the exam.