Bull Peter D. Bull 
Vengeance Is Ours [EPUB ebook] 

Harry and Jack owe money to O’Brian, a violent gang boss. They’ve spent his money, so have to hide in Amsterdam. Jack falls for a beautiful prostitute. Harry returns when told of O’Brian’s death at the hands of the Rossi crime family. Harry and Terry (his brother) meet Jack on his return. Whilst at a brothel they’re faced by one of O’Brian’s gang. Terry kills three of the Rossi gang. Whilst on remand, Terry is killed by the Rossi’s. That starts a vendetta, "Vengeance is Ours" is the mantra as Harry an Jack take revenge.