If you are looking for a good read with excellent pictures, you should take
a look at the following site, which was introduced to me by Andrew Simmons of
Warrenorth Nursery here in England.
The site belongs to Marilyn Holt who runs a mail order nursery in Canada.
There are over 1500 pelargoniums with descriptions and a number of photographic
references. The site has just been upgraded and must be one of the best sites