I should be going to Geneva in the Summer.
Am I going to be OK with my "Birds of Britain and Europe" (Hume), or is there a more localised (but English language) bird book I should look out for?
The only problem that you may have with a guide to the whole of Europe is that the maps won't be a suitable scale to clearly show what is likely to be present around Geneva (although you should be able to get a pretty good idea!).
I would suspect that you will find it hard to buy a field guide to just Swiss birds that isn't in French or German.