In the Best Families — Rex Stout

The aging millionairess has a problem: where is her young playboy husband getting all his money? To help find the answer, Archie infiltrates a party at her palatial estate. But her late-night murder ruins the festive mood...and a letter bomb from a powerful crime boss makes Nero Wolfe do the unthinkable--run for his life. Suddenly Archie finds himself on his own, trying to find a killer without the help of his old mentor. For to all appearances, Wolfe has vanished. The career of the world's most famous detective has ended in cowardice and disgrace...or has it?