Imitation may be the most sincere form of flattery, but it's not going to serve you well in the publishing industry. Regardless of the topic, genre or length of the book that you plan to write or have already written, an editor's expectation is that whatever you submit for review is your own work and not a rip-off of someone else's material. While you can certainly have been inspired or influenced by other writers' novels, plays or movies, stealing from them and claiming credit is plagiarism.
In the case of nonfiction works that require substantive research or quotes that you obtain from interviews, it's essential you identify and properly credit your sources in the body of the text or a bibliography. Most publishers require signed releases from individuals who provide interview content and/or photographs for your book and which specify the terms under which permission was granted. Examples of this include a fee, a percentage of royalties, a copy of the book, or a thank-you in the foreword.
Before you start shopping your book to buyers, it's important to register it first with the US Copyright Office and establish its completion date. This can easily be done online and protects the work for the life of the author plus 70 years. A common misconception about this process is that it automatically entitles the author to sue anyone who writes a similar book. In truth, a lawsuit can only be pursued if solid evidence exists through a paper trail that the party not only had direct access to the material but appropriated major sections of it verbatim. Another misconception is that you can copyright an idea or a title prior to writing the book. Stand-alone ideas and titles cannot be registered. (See Resources.)
No one likes to have nasty things said about them in print. Even if your ex-husband is the biggest jerk on the planet, writing a vicious tell-all about his drinking, gambling, skirt-chasing and cheating on taxes will likely get you a visit from his lawyers. Yes, you told the truth, but the courts will heavily scrutinize what your real motives were for doing so. Nor can you liberally trash public figures just because their lives seem to be an open book. Even dead people have rights--as well as live descendants and heirs--if you paint them in a slanderous light just because you think they can't get back at you. If in doubt about what you can and can't do, always consult with an attorney.
Once you sell a book to a publisher, you can't sell the same book to someone else. This is spelled out clearly in the contract you sign along with the particulars regarding the amount of your advance, the royalty structure, how the rights will be negotiated insofar as adaptations and translations, and under what terms copyright will revert to you. Also included in these recitals is the prohibition against putting any of your book's characters into a new book and selling it to a different publisher. Interestingly, the popularity of ebooks and self-publishing entities has set forth options allowing authors to not only keep all rights to their books but set their own prices and royalties as well. Contracts vary, of course, and should always be reviewed carefully.
In the event you decide to use an existing book as your source of inspiration and give it a fresh spin, you cannot do so unless you have written permission from the copyright holder. The exception to this is if the book is in the public domain. What this means is that the work was created prior to existing copyright laws in 1923 or its copyright registration expired. (See Resources.)