tapply

I'm having trouble with tapply, it seems to give me NA for the mean of a set 
like {2, 3, 4, NA}, can I get it to ignore the NA's and give me a mean like 
say, three? Am I just doing something stupid?

Any help would be apprecitated, Also are there PDF versions of any of the R 
stats books around that I might be able to get my hands on?

Bryn