The right publisher for your book is the publisher who wants it and can do most for it. Homing in on that publisher is not always a straightforward process and you may have to go through one or more rejections before getting accepted. The more accurately you target the publisher, the less likely you are to get rejected, so when selecting one consider the following points:
Did your supervisor or examiners suggest a publisher? Did they provide a named contact? Follow it up.
Does your department or university have links to a publisher or even run a university press itself? Check whether they might be appropriate.
When reading recent literature on your subject, did you find you were reading more than one book from a particular publisher or more specifically from a particular list (a list is a series of books by different authors edited by one or more people on a particular topic, eg Medicine and Culture edited by Sander Gilman at The Johns Hopkins University Press)? Perhaps this list would welcome your research: find out as much as you can, from the books themselves and from general statements on dedicated webpages, about the list editor's publication policy. Perhaps approach the series editor directly via email or letter.
Think about what YOU want for your book, what sort of readership, circulation, prestige and price you want for it. A small academic press may be unknown by the general public, rarely place its works for sale in bookshops, yet still be stocked by all research libraries in the world (eg The Voltaire Foundation in Oxford which publishes monographs on the Eighteenth Century - SVEC). Alternatively, it may not enjoy much prestige or do much promotion, so that their books receive little attention: they would really be a last resort. On the other hand, presses which are highly visible and sell cheaply (at least compared to SVEC) in bookshops may rarely get to a non-anglophone readership (Routledge may be one such). You need to decide what is important.
Scour printed publishers' catalogues and websites for description of publication policy and interests. Here are a selection, necessarily incomplete (we welcome suggestions: contact us!) of anglophone publishers that include lists in the humanities and in modern languages.
Cambridge University Press (UK)
Chicago University Press (US)
Columbia University Press (US)
Cornell University Press (US)
Duke University Press (US)
Edinburgh University Press (UK)
Edwin Mellen Press (US, UK)
Johns Hopkins University Press (US)
Manchester University Press (UK)
Oxford University Press (UK)
Palgrave Macmillan (UK)
Peter Lang (Germany, Switzerland, UK, US)
Routledge (UK, US)
Stanford University Press (US)
University of Toronto Press (Canada)
Voltaire Foundation (UK – dedicated to Eighteenth Century studies)
Yale University Press (US)
Check the journals you most commonly read: what press are they affiliated to?
Check their review section: who is publishing what?