The issue of how far to stray from one's doctoral research (now known as my forthcoming book) is something I have given a lot of thought to. Partly because I chose to head off into a very different direction with my next (current) project.
A few historians, wiser than myself, have discussed the issue. This week Notorious, PhD brought in the Fox and Hedgehog analogy - parts 1 and 2. The idea is that the fox can do many things, while the hedgehog does one thing and does it well. The historical fox changes period, or region, as they move from project to project, while the hedgehog becomes a focused expert on one specific place in time.
For a variety of reasons, I (perhaps naively) never thought of my doctoral work as my magnum opus. Still less that I would be constrained to spend the rest of my career drilling ever deeper into one postage-stamp sized piece of global history. I thought of it more as an apprentice piece in historical training. Much as an apprentice furniture maker might make a beautifully turned chair as his or her apprentice piece, but this does not prevent him or her accepting a commission the following day to make a Welsh dresser.
So I have been impressed rather than skeptical when a big-name professor writes a book on a new field. Since we all originally wrote a dissertation from scratch in 3 years, why shouldn't someone who has completed that training be able to do it again in another area? (or indeed, take less than 3 years, since they have developed better research skills and possibly have access to research assistants). I rather take the view that any trained historian should be able to turn their focus to historical study of anything, at any time (the main limitation being language barriers).
But there is resistance from some quarters (at least in the English-speaking academic world) to people who seem to stray too far from what their PhD "allows" them to research. There are indeed people whose entire careers seem to be reiterations of their doctoral work, with their research on Silk Weavers in Podunk County, 1850-1855 being complemented by the later work, Silk Weavers in Podunk County, 1855-1860, and perhaps a broader study, like Silk Weavers in the Podunk Valley in the Nineteenth Century. This is contrasted by the expectation in Germany, where the habilitation (essentially, second PhD that qualifies someone for a university position) has to be on a markedly different area from the doctoral work.
My Masters research was on people going native, and their representations in culture. My PhD was about women in East Asia, and the nature of modernity. What I am looking at now still focuses on modernity, in the urban setting, and the nature of culture-genesis in colonial sites. I think of myself as a cultural historian, and one thing these projects clearly have in common is the idea of cultural exchange and influence. "Fusion" history, you might say! (unfortunately, not that many jobs seem to be going for professors of historical fusion...).
I am still too young in the game to know whether I made an inspired career choice or a colossal error in choosing the path I did. But I absolutely love what I am researching now, and it makes me happy. What has become most clear to me as I look through my research notes, and find myself using things I wrote down years ago - not because they were pertinent to the project at the time, but because somehow they seemed relevant and worth keeping - is that I am neither a hedgehog or a fox. I am a squirrel. I seem to have a habit of finding useful sources and connected ideas, and putting them somewhere safe for me to go back to in the winter.
This was me as a baby.
(image from cuteoverload.com)