Make sure your reference book manuscript is ready to be published. This may mean hiring a professional editor, copy editor and proofreader to go over the manuscript. There is no point in taking the next step to approach a publisher or to self-publish until the manuscript is perfect and contains everything you want to say in the way you want to say it.
Decide whether to go the traditional publishing route or to self-publish. There are pros and cons of both choices: with traditional publishing, the editorial services of book design, copy editing and proofreading, as well as the major task of marketing, come with the package. On the downside, the author maintains less control, and the royalty structure is usually lower. With self-publishing, the author may have to take on all these tasks himself or hire someone to do them but has total control of the book. The royalty structure is usually better in self-publishing, and there are no intermediaries, such as an agent or publishing company, to split royalties with.
Find a book agent. This is not absolutely necessary, but very helpful should you find one. The best way to find an appropriate agent for your particular book would be to look at similar titles that have been published, and look for the agents or agent companies representing that writer or subject for a clue as to what kind of books that agent is looking for. The online search engine agentquery.com is an excellent resource for this.
Approach publishers. You don’t need an agent to approach a publisher. For traditional publishing, some of the companies that publish non-fiction titles in the reference category include Sage Publishing, Altamira Press, Bernan Press, Government Institutes Press, Rowman & Littlefield, Scarecrow Press, Jason Aronson, Hamilton Books, Lexington Books and the University Press of America. In the self-publishing realm, the major houses include CreateSpace, AuthorHouse, Lulu.com, iUniverse, Llumina and Xlibris.