NYPD Commissioner William Bratton said he plans to plunk down $28 -- $18.41 if he goes for the online discount -- and buy his predecessor Ray Kelly's new book: "Vigilance: My Life Serving America And Protecting Its Empire City."

"I think Ray's book will contribute significantly to an understanding of the history of what has been going on in this city for the last 25 years," Bratton told reporters Monday, noting that Kelly's book will be helpful in giving perspective to some of what happened in the city for the past two decades.

But Bratton took issue with Kelly's assessment, reported this weekend in the New York Post, that Mayor Bill de Blasio has negatively impacted public safety, in part because of the way he campaigned against stop and frisk and the police in 2013.

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"Crime is down for the second year in this mayor's term of office," Bratton said. "So I don't see that concern reflected in reality."

On stop, question and frisk, Bratton admitted that he and Kelly sharply disagree, notably on the issue of the large amount of such actions done under Kelly.

While crime initially went down under stop and frisk, crime continues to go down even when dramatically fewer stops are being made, Bratton said.

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"The belief that crime was associated exclusively with [stop, question and frisk], that is not correct, it is an element certainly like anything else that we do, but it was not a dominating fact," Bratton said. "What it was dominating in was clearly creating a very significant rift, if you will, between people of color and this department."

Bratton also said de Blasio has been the most supportive mayor he has ever worked with, giving him resources and money to do the job.

"In everything I have asked of this mayor . . . he has responded," Bratton said.