If you already know the answer you can probably find
the answer in wikipedia, but I would not recommend it
(or any encyclopedia) as an introductory text.
The first hit is
physics.ucsc.edu/~peter/116C/helm_sp.pdf
which seems OK to me at first glance, although perhaps more
of a review than an introduction. At least it tells you what
to look for, if you want to continue looking.