Research. Movies and popular culture have distorted many impressions about organized crime, and anyone who wishes to write about them needs to separate fact from fiction. This isn't to say you can write a lurid, fictionalized account of mob life; only that you understand where and why you're departing from the facts. Study the development of organized crime culture, the particular rackets it engages in, the social structure it uses and the means they have of maintaining order within their ranks. Knowledge of mob culture lends authenticity to your prose, allowing you to speak authoritatively about the topic.
Specify the kind of mob you want to cover. The traditional American Mafia isn't the only type of organized crime in existence. The Japanese yakuza, the Russian mob, the Columbian drug cartels and similar entities all fall under the definition of "mobsters." They each have their own sphere of control, unique subculture and areas of interest that separate them from "classic" mobsters. If you wish to focus on one of these groups, study the specific modus operandi of each and apply that information to your book.
Decide if you want to write a fiction or a nonfiction mob book. If you're writing nonfiction, organize the information you intend to include, divide it up into chapters and be prepared to cite the sources of any assertions you make. If it's fiction, develop a series of protagonists, flesh out their position in the mob, determine their relationship with those above or below them in the ranks and develop a plot line exploring their growth and progression through a series of (likely criminal) incidents.
Pen a draft of your book, based on your outline in Step 3. Include every piece of material pertinent to your story or subject matter and don't worry about edits or revisions yet. Simply get it all down on paper, from beginning to end.
Revise your first draft. Improve the prose of shaky passages, streamline the flow so that it moves at an engaging pace and excise any areas that bog down the text. Check for internal logic if you're writing fiction and source citation if you're writing nonfiction. Double- check all of your mob-specific details and make sure the information you provide is accurate (or, if you're embellishing, you understand why you're embellishing and what affect you hope to achieve).
Give your text to a trusted reader after two or three revisions. It can be an editor, a spouse or a friend who you trust to deliver honest and constructive criticism. Ideally, it should be someone who doesn't know much about mob culture, to point out any areas that require clarification. Take their comments to heart and use them to perform further revisions.
Finalize your text and contact a publishing company to see about getting it published.