This is a scholarly(-ish) study of the shift towards professionalism in police forces in the United States. It was originally published in 1967, so is quite out of date as far as police studies are concerned, but still offers a snapshot of that field at an interesting time. It does come from an insider, but rather than defending the status quo, the author comes down on the side of reform.
But perhaps what is most interesting is the measurement of attitudes that are revealed by a survey the author cites about who cops hate the most. Something suggests that not much has changed in that regard. At the top? Cop-fighter.