This website has a lot of really helpful information.
http://www.zerotothree.org/site/PageServer?pagename=key_l...
Also, my son is 15 months and says 2 words, but knows more than that, he just doesn't say them. He started walking at 9 months though. I read to him everyday, and teach him words and colors, shapes... but he is developing at his rate. He falls into this category:
"Late talkers
Babies often specialize in one area of development at a time. So between 10 and 11 months, one particularly persistent boy spends nearly every waking moment trying to walk. Once he takes his first independent steps, he is all over the house, walking for the sheer joy of movement. He has little time to think about words for all of the things he is seeing. By 14 months, the walking is less all-consuming, and he begins to point at things, as if to say "what's that?" His vocabulary picks up after that."
http://www.drspock.com/article/0,1510,3920,00.html
Children develop at different rates, so I wouldn't be alarmed that your son's vocabulary isn't impressive, as children learn to tackle many things at once. It's the babies that are slow in development in all areas that really need professional intervention.
But of course, no matter the age of the child or the rate of development, reading and teaching and learning activities are a wonderful thing.