Hi
One of the best illustrated guides I have found is "1001 Pelargoniums" by Hazel Key published in 2000. The ISBN is 0-7134-8268-0. Unfortunately this book does not have an index.
Another book which is also very good is "Geraniums" The Complete Encyclopedia by Faye Brawner The ISBN is 0-7643-1738-5. This is an American book and may be easier to obtain from a library.
Both books contain a great deal of useful infomation.

Best wishes

Ron

If it's hardy Geraniums you need either Hardy Geraniums by Peter Yeo or Hardy Geraniums by Bath and Jones