The First Rule by Robert Crais

Frank and Cindy Meyer had the American dream – until the day a professional robbery crew invaded their home and murdered everyone inside. The only thing out of the ordinary about Frank was that – before his family, business, and oh-so-normal life – a younger Frank Meyer worked as a professional mercenary . . . with a man named Joe Pike.

The robbery crew has done other home invasions, and all the targets have been criminals with large stashes of cash or drugs. The police believe the same is true of Frank, but Pike does not, and with the help of Elvis Cole, he sets out to clear his friend . . . and punish the people who murdered him.

They are about to learn the first rule.

Don’t make Pike mad.