All instruments, including the human voice, produce frequencies
in addition to the fundamental. They are almost always of less
magnitude than the intended note (although the very lowest notes
some pianos are said to consist of more total harmonics than the
fundamental).

The other frequencies are "harmonics" and can be above the
intended note or below it ("sub-harmonics").

The exact combination is called a "format" and is the reason we
can tell an oboe from a violin, or even one violin from another.
No matter who plays it, a given instrument played at a certain
pitch will produce approximately the same quantity of harmonics,
giving that instrument it's characteristic "timbre".

As a biologist, I have no books on this interesting subject.