Research and identify the main, competing books in your field of expertise. Compare your book for original interpretations, new ideas and redundancy. Through this process, identify your target market.
Write a proposal with a detailed synopsis. The synopsis includes a chapter-by-chapter description of the book. You also want a strong marketing plan and author biography. The biography is focused on your relevant credentials.
Attend academic conferences in your field to meet editors. Learn what topics publishers are interested in at the time. Pitch your ideas to editors you meet and ask for feedback, which can interest an editor in your ideas before you send the full manuscript. You can also search calls for publication at well-known sites like H-net.org. Calls for publication are announcements by publishers about what kinds of books they want to publish.
Send your proposal to editors who expressed a serious interest in your ideas, but also focus on major academic publishers such as Oxford University Press, University of Texas Press, Cambridge University Press, Johns Hopkins, SUNY Press, Routledge, Kluwer, Taylor & Francis and John Benjamins.