The most important rule of any research proposal, especially of a PhD proposal or a proposal for a book, is that the subject should be interesting for the researcher. You will spend much time in libraries and archives. Much of the research may be boring: lots of time spent on collecting data, removing dust from old documents in archives and reading stuff that is not always that interesting (although sometimes it is very interesting). It is, of course, ideal to enjoy the process. However, when the process is not that interesting at certain phases, it is good that the subject in general is interesting for you. It helps you keep going when things get though.

And things get though at many stages. One can always fail. You never really know if you will discover something substantial. So remember, when you choose a subject for your PhD, book or article, make sure that you feel close to the subject. There are people who do research on things that they do not like. Yet, it would be better that you subject will be on something that you love.