This is an old book, published in 1963 and part of my collection of Companion Books. So I must have read this before, but I confess I don't remember it.
   I do remember other books by Ernest K Gann. In the Company of Eagles, for example. He's always been one of my favourite authors, and Of Good and Evil did not disappoint.
   It tells the story of a city police force and some private citizens, over the course of just one day. It's brilliantly done, and the book is sufficiently old that some of the laws and prejudices of the time are plainly on view. The city, according to the introduction, is probably San Francisco, although that is not explicitly stated in the book.
   If you get the chance to pick this up in a secondhand bookshop, go for it. It's a rare treat.