This was only the second of the Edgar Best Novel winners so far that I knew for certain I had read before. But, I decided it would be worthwhile to reread it, and how right I was. Martin Beck, the protagonist of this series, is the spiritual ancestor of Henning Mankell's Kurt Wallander. He pretty much bears out any stereotype you may have about gloomy Swedes. But he's a heck of a policeman.
One thing I don't recall noticing when I first read this book back in the 1970s was how it is set in a definite time -- 1967, with protest demonstrations worldwide about America's involvement in Vietnam. The book opens with such a demonstration in Stockholm, with most police detailed to keep order. Shortly, however, Beck is called to a crime scene -- someone has shot all the passengers and the driver of a city bus. And one of the victims is one of his own homicide detectives.
The solution of the case leads to the solution of a "cold case" from the early 50s, and owes more to good, solid, routine police investigation than to any stunning intuitions on the part of Beck or his colleagues. (As is my wont, I had forgotten "whodunnit" long ago so that I enjoyed not only the writing, but the mystery.) I very seldom reread mysteries, but the Sjowall and Wahloo series is well worth a reread, or a first read if you haven't encountered them yet.