have a look at the Hardy Plant society [HPS], they ahve many branches all over Britain. You might find one near to you. also look in your local library for gardening clubs. I live in a small village and there is one here too. They also do small books /monographs on a range of different plants. They do do an Iris one but dont know if it is all the irises or specific ones.

I don't know of a book specific to Iris Germanica. The nearest that I am aware of, and currently in print, is 'Irises' by Clair Austin (Timber Press £35) and 'The Iris Family' by Goldblatt and Manning (Timber Press £40).
For local enthusiast groups try THE BRITISH IRIS SOCIETY, www.britishirissociety.org.uk. They have a very informative web page and there are local groups around the country. They may even do their own publications.