I wrote a longer paper on exactly that topic last year, so I hope my input will help you a bit at least.
Depending on the length of your dissertation, I figure it's always better to rather pick few primary texts and analyse them thoroughly and in much detail, than having lots and analysing them broadly. How long is yours going to be? Three should be fine, though. If you want to look at one less obvious example, T. H. White's Book of Merlyn might be interesting for you. There is one chapter where King Arthur is transformed into an ant and goes to work in the ant society, which is like a perfect allegory on Nazi Germany.
As for secondary texts, too many is better than too few. For relatively short dissertations, one rule of thump is using one secondary text for each page of your dissertation, however this wouldn't make sense in long dissertations.
As for narrowing the topic down, I think your topic is OK like that already. The texts you use were all written in early 20th century, so why not put it like that, 20th century Dystopian fiction?
Regarding structure... You could start the main part of your dissertation with a general theoretical part on Totalitarianism, i.e. describing possible origins, its main characteristics and in the end come up with a definition of your own. I know you could write books on that topic, but if you summarize it well, this can work perfectly. Plus, there are many good texts on the nature of totalitarianism. Ahrend is a good start.
After that, come up with your analysis. There are two ways you can structure that. You can either start off with the analysis of one book, then continue with the second, finish off with the third and in the end compare them. Or, which I think makes more sense, you structure this by the main aspects or results of your analysis, i.e. you make short chapters on the different aspects and in each chapter, compare how the given aspect is portrayed in the individual books, for example 1. Control of the mind 2. Establishing comformity of the individuals 3. Mass propaganda, etc. The disadvantage of this approach is that you might lose some of the features that are only described in one of the novels and not in the others. With the first approach, however, you are in danger of repeating yourself and might struggle to find a representative result.
Last edited by qua; 13-06-2012 at 10:11.