I have no direct answer for you, aside from there is much more likely research and publication about individual European countries than there is about Europe as a whole. Possible research directions (in random order):

• Spain had a fascism problem until a few decades ago
• France is relatively hierarchical, violent riots against the government are more normal there the in other European countries
• the UK and Ireland had religious conflicts concerning Northern Ireland, aka the troubles
• large parts of Eastern Europe where opressed by the USSR, speaking out was not safe everywhere.
• Former Yugoslavian countries are still at odds with each other, freedom of speech and independent media are not a given everywhere.
• Germany has a strong allergy for secret services, doesn't want government or corporations collecting private data
• Italy had a maffia problem (they still do but it's much less influential on the everyday lives of regular people)
• Many European countries have a group of right wing voters that are increasingly extremist, violent 'protests' happen more often now.
• If Europe includes Turkey, the (failing) independence of the judicial system there might be worth reading about. Even if the police is kind to you, it means nothing when a judge can put you in jail just because the 'boss' asks him to.

I think if you're looking for stories about police brutality in Europe, your best bet might be to look for events involving riots and protests. Deaths like those of Renee Good, George Floyd or Eric Garner happen much less here afaik, but they do happen. This is not a book, but there is a foundation in the Netherlands that focusses on fair law enforcement (not specifically against police brutality) and they have kept an archive (for a decade now it turns out): https://controlealtdelete.nl/dossier-politiedoden?lang=en#gsc.tab=0. If such an organization exists in other European country, there could be large differences in the data.

One big difference is that the education to become a police officer in a (or most?) European country is much longer than in the US, another big difference is that barely any military equipment ends up with the police. I think those two, combined with racism and loyalty to co-workers, are the biggest problems when it comes to police brutality in the US (i say that as an outsider though). Europe might be equally racist (not sure) and equally loyal (again not sure), but i believe our checks and balances system are generally working better at the moment; which would mean the jerks are more likely kicked out and abolishing the entire organization is much less needed to 'fix' the police.