Each of NYPD’s last four commissioners has written books, most of which are filled with hot air and self-promotion — and in the case ofBernie Kerik’s autobiography “The Lost Son,” maybe some alternative facts.

Now comes “Blue on Blue: An Insider’s Story of Good Cops Catching Bad Cops,” from former Chief Charles Campisi, a 41-year veteran who headed the Internal Affairs Bureau for 17 years. It’s probably as honest and down-to-earth an account from a NYPD official as we’re going to get.

Campisi, who retired in 2013, loves the NYPD and represents the best of what it stands for. Yet, his views on a couple of issues are opposed to those of the public.

As IAB chief, Campisi did not experience a major corruption scandals — at least not traditional corruption. Instead, the NYPD faced perhaps the two most brutal acts in its history, both of which he investigated.

First, Haitian immigrant Abner Louima was sodomized in a 70th Precinct bathroom by Officer Justin Volpe in 1997. The second was the 41-bullet fusillade by four cops that killed the immigrant Amadou Diallo in the Bronx in 1999.

In the Louima case, IAB broke the so-called Blue Wall of Silence. In the Diallo case, the public might have trouble understanding Campisi’s defense of the cops, one of whom mistook Diallo’s wallet for a gun.

The police union pushed to have the trial moved to Albany, where the cops were acquitted. The union called the shooting “a tragedy, not a crime,” a point of view Campisi echoes.

The public also may question Campisi’s defense of Ray Kelly, under whom he served for 14 years, and Kelly’s stop-and-frisk policy, which a federal court ruled was illegal. Here Campisi pulls his punches, suggesting former Commissioner Bill Bratton was responsible for the illegal stops during Kelly’s tenure.

In his first term as commissioner in 1994, Campisi writes, Bratton insisted cops “be more proactive in conducting stop-question-frisks . . . As a result, the number of stop-question-frisks surged — by 2012 the NYPD was making more than 500,000 stop-question-frisks a year — and crime had plummeted.”

Those are alternative facts. Crime began to drop before Kelly returned as commissioner in 2002, and the more than 500,000 stop-question-frisks occurred not under Bratton but under Kelly.