Research which publisher would be the best fit for your novel. Established romance literary houses, such as Harlequin Romance, have been around for a long time and are extremely picky about the kinds of books they publish. Some publishing houses only will accept a limited number of novels a year, and some publishing houses only will accept books that are represented by an agent or from certain genres. Know to whom you want to send your book, and what that publisher is accepting before you send it.
Prepare a query letter. Numerous sites on the internet give examples of what makes a good query letter. A query letter is an introduction between yourself and the publisher or agent with whom you want to publish your book. Publishers will have guidelines on how and when to query on their website and in publishing directories. Each query letter is different for different places. Some require the first chapter of your manuscript, some only want a synopsis and some require the whole novel. Some publishers only accept queries by email, and some only by snail mail. Make sure you find and follow their guidelines before submitting your query.
Consider a literary agent. A literary agent's job is help you find the right fit with the right publisher. Beware of agents who ask for up-front costs; an agent should only get paid when you land your publishing deal, not before. A good agent will help you market yourself to people who may want to publish your book. Some publishers won't consider a manuscript unless you have an agent. One way to look for agents is to find out who represents some of your favorite authors, or other writers who are successful in the romance novel industry.
Self-publish. If you can't find a publisher or an agent to work with, self-publishing is always an option. You retain all rights and royalties to your book, but you are also solely in control of its marketing. There are several places where you can publish online, and fees vary, depending on company, binding and distribution.
Try turning your novel into an e-book. An e-book is an electronic book that can be downloaded straight from a website, or a huge distributor and usually compatible for different electronic reading devices such as the Nook, Kindle, or Sony e-reader. Many books today are published in both their paper and electronic forms at the same time. Research which e-book publisher would be the best fit for your novel. Self e-book publishing is also an option.