I have come accross a Weather Watchers Observer Handbook
It is very good and it is free to download from the link below.
It covers many topics including:-
Observing the Weather
The Weather Station
Temperture
Visibility
etc
Excellent find!
I’ve added it to my library of PDF’s.
Information about snow measurement is usually hard to find - lots of options in this handbook.
a few good other sites are
http://www.campbellsci.com/siting.html#rh
http://www.nws.noaa.gov/om/coop/training.htm
this one for snow
http://www.nws.noaa.gov/om/coop/snowguid.htm
actually I think i will add it this as a topic in the FAQ 
Another good basic resource is the USA Today Weather Book, most of which is online here http://www.usatoday.com/weather/resources/basics/wworks0.htm