I want to make it clear from the beginning that the word hunt in the title is not used in the most common sense, so do not think this:
One of the recurring characters of the series is Archie Goodwin's girlfriend Lily Rowan. She hired a secretary Amy Denovo for a temporary position who asked Archie to help her find her father one day. Amy grew up with her mother who never told her daughter about her father before she was killed in hit-and-run accident. Amy did not want to involve Nero Wolfe because she thought Archie working by himself would be cheaper and she could not afford Wolfe's rates anyway.
Archie almost never works for himself, so he refused. The very next day Amy showed up in Wolfe's house with big chunk of money in cash to pay Wolfe's retainer. Asked where she got it she said this was money from her father about whose identity she had no clue.
I already mentioned it elsewhere that Rex Stout used more social commentaries in the later books of the series. This time the more the detectives learn the background story the more it becomes a sad tale of a power the rich have over little people. This time a millionaire practically destroyed a woman's life who really could not catch any breaks afterwards. This is probably the first time ever when Wolfe delivered (as if there were any doubts about it), but his client left completely unsatisfied with final results.
I just want to make one more remark. I saw people give lower rating to this novel for the reason it aged. Their argument: in modern time we can simply use a DNA test to avoid all of the problems that the detectives faced, so the book aged, so it deserves a lower rating. I can only say to this, "WTF?" Sure they had not developed DNA test at the time of writing, but does it mean it is hopelessly outdated? In this case why not criticize any book featuring sword battles for not using automatic rifles, or Jane Eyre writing letters to Mr. Rochester instead of just sending him text messages? Does this look ridiculous? So does the original criticism. Rant mode off.