I'm two years out of my PhD in the humanities and have given a couple of job talks since, all on material from my dissertation. Not only have I spent months trying to make the talks themselves as good as possible, they're based on ideas and material I've had the entirety of my PhD to feel confident about presenting. My question is: at what point on the market do universities expect you to present new material that's not part of your dissertation research? I'm at the very early stages of developing my "second book project," but my research / argument aren't nearly as developed!
I’m in a vastly different field, but if your work is progressing at approximately the same rate as your community’s then I wouldn’t worry about it.
Perhaps you can talk about your ongoing work as well just to signal that you’re not stagnant. The point of the job talk is to assure the department that you’re going to do good work while there!
Most universities want to see that you are at the publication curve or ahead of it. For example, its perfectly ok to talk about your dissertation three years out of grad school if you are talking with your now published results. If, on the other hand, you are discussing a phd that you never turned into a publication, that can be a little trickier. If you are currently in the revision process of the publication, thats probably ok too.
The main point that I am getting at is that talking about your dissertation is ok, its likely the foundation of your future research. Its even better, though, if you are discussing the published results from your dissertation in a journal or in your case, your published monograph.
I do not know how it in humanities, but in general the idea is that in job-talk you present the solid work that you have already done and preferably published. Put it simple: you should present the most solid work that you have done because it is the track-record that you are judged. The talk can include some portion of newer work, but as soon as it is not published it is better to be only small part. In job-talks there is a common practice to devote 5-10 last minutes to future directions. Your new part can go for example there.
If you are already in a tenure track position, you should start talking about new work now. Maybe make it a blend if you just don't have much. But certainly I would not emphasize earlier work just because you have a more polished talk.
If you are still looking for a good position (out of work, or doing post docs or lecture positions) than it is fine and maybe even preferable to concentrate on the thesis. But even here I would emphasize some of your recent work.
Remember these talks are not just talks on the content and for enjoyment but advertisements of you. Yes, make them pleasant and revealing, sure. But make yourself look good.
For as long as the material is relevant.