Now and then a faculty member will come to one of the summer scholarly book publishing groups with a nervous message for me. It often goes something like this: "I told my friend what you told me about querying university press editors while you're working on a book, and she said you can't do that, because if they ask for the whole manuscript and the book isn't finished, they'll be pissed."
I hope these faculty members forgive me when I laugh before I answer. Hmmm, you can't do that? Or "they" will feel a particular way? Like Dick Van Dyke said in the old 1960s TV show, I want to ask "Who's this they?"
In reality, querying editors while you are working on a scholarly book can be a powerful way to gauge interest early without committing to one press, and also of getting helpful feedback. I originally got the idea from speaking to university press editors themselves. Two key lines will help you as you do this. I like to say "This is an inquiry only, not yet a full submission," and "I anticipate completing the manuscript by x date," so that nobody accidentally imagines that it is sitting on your desk ready to go.
By the way, scholars, the trade press world snickers when they hear that we have been taught to completely write our books before submitting them. That's a little like designing a suit without knowing what show it is going to be in. Most university press editors are more proud of their publishing lists than they are of their own children, and even though your work is uniquely you, it also has to work on one of those lists. It will be shown as part of a set, not as a one-off.
Also, the editors I have met tend to be intelligent, collaborative, and brimming with good ideas about what might make books work and why. They've been through this a time or two, so why not give some of them the opportunity to communicate early? You just might end up with a deal before you finish your book, and wouldn't that be a comforting way to spend the last months of your writing?
Stack o' books above is the work of Czech artist Matej Krén, whose terrific work using books as a medium can be gawked at here.