A.C.
This is a good, short picture book on the topic:
Your body belongs to you, by Cornelia Spelman
It talks about how only Mommy, Daddy & your doctor are allowed to touch your private areas (as in, covered by your bathing suit), and that you don't always have to or want to hug/kiss/etc. people, even if they want you to. (At least, that is my recollection; we read it over the summer, so my memory is a little fuzzy). Our library carries it, and we will be getting it out multiple times over the next couple of years (he's in preschool now).