Guys have always traditionally been interested in gross or weird bodily functions, and we have always provided, at least for younger kids (see Ripley's Believe It or Not et al.)  The fun doesn't stop there, though.  There are plenty of questions that remain that aren't really covered by those slightly more kid-friendly titles, ones that we aren't really likely to present to the younger crowd, like the titular question.  It's not that it's a bad question, but it can be awkward.  After all, while there is nothing wrong with feeding one's baby, it is not a comfortable subject for everyone.  But, well, here: some men have indeed put them to use. 

Why Do Men Have Nipples? is an entertaining read aimed at adult audiences written by an emergency physician and a humorist, so the topics are approached with a pretty relaxed attitude.  They are not entirely family-friendly, but I wouldn't worry too much about giving them to older teens.  This is factual, educational stuff, and in some cases could encourage young readers to take up medicine.  The human body is weird.

Learn why asparagus makes your pee smell.  And why can't some people smell it?

Why do old folks have hairy ears?

What's the deal with ice cream headaches?

There is also a follow-up book, Why Do Men Fall Asleep After Sex?, covering more of the same, with an added touch of the Battle of the Sexes.  These are older books, but they are great examples of well-focused trivia books, and they aren't too out of date.