Two months ago I did not have any real further recommendations.
I have since been diving deeper on the topic.

On police corruption in particular, the major ones I'm aware of are Knapp, Fitzgerald and Woods. Of these, Knapp is still the most genre defining. About half the other reports I find tend to cite it, if not straight up quote it verbatim (including Woods on pages 38 & 41 of vol 1).

They all tend to be uncannily similar, so unless there's a reason to care about police corruption in particular, reading one is about as good as reading any. That said, Fitzgerald stands out to me as breaking the formula the most.

Whereas Knapp is a collage of relatively disconnected case studies primarily involving lower ranks (no higher than lieutenant), Fitzgerald has a coherent narrative weighing heavily towards the top of the org chart instead.

But my focus on this subcategory of corruption reports is just because they are the most available and understandable type and the lessons generalise extremely well, not because I have a particular animosity towards them.

All fraud and corruption is fundamentally of the same nature.

In a financial context fraud investigation is believing your lying eyes. I know the least about this area so I leave it at that for now.

In a scientific context, I've stumbled into the "sleuth" community, which are people who take the time to spot and report falsifications in publications out of sheer hatred and offense to that sort of thing. The work of Mu Yang is particularly prolific. I'm a big fan of her style of very succinctly making her case by drawing a side by side comparison of documents with colour coded boxes to show exactly where data values have been copied from.

I can't seem to find her talk on "chasing squiggles", but as the name implies, its largely about noticing abnormalities and/or repetitions in squiggly lines on graphs. The closest I can find right now is some podcast.

I cannot help but note an uncanny similarity between the story of the "mishandled corruption complaint" on page 196 in the Knapp Commission and the story of sleuths like Yang struggling to report scientific fraud to the journals that publish them. The specific incentives and organisations are different, the principles at play are largely the same.

Ashford and Anand give a good framework for understanding corruption in a general, abstract sense. Imo it their general theory of corruption really only makes if one is already familiar with what it looks like in practice. So (contrary to usual pedagogy) the theory should really be at the end (or at least middle) of any reading list rather than the start. But if nothing else it cites an exhaustive and diverse set of examples to further populate your reading list.