Write your own book. Novels or self-help books are always popular. Pro book publishers will pay sizable advances for projects they believe will be a profitable fit for their company. You'll also collect royalty checks on your published book. Once you've written a novel or book of nonfiction, use a writer's market database to find a publisher who will handle your book (see resources.) Write a query letter introducing yourself and giving a short synopsis of your project, then wait for the publisher to tell you how to submit.
Consider self publishing your book as an e-book. There are a number of publishers who will accept books in electronic book format and publish them on their websites (see resources.) Some of them charge a percentage of sales and others are free. These e-book publishers will keep track of your earnings and send you quarterly royalty payments either by check or through Paypal.
Ghostwrite a book for someone who has an idea for a story but doesn't know how to write it. The downside of this is that you don't usually get any credit for the writing, but the upside is you get paid. Either negotiate a flat rate for writing someone's book for them, negotiate an hourly rate, or work for a percentage of sales. Consult Craigslist or a website that advertises work for freelance writers to find someone who needs a ghostwriter (see resources.)
Enter into a work-for-hire relationship with a publisher. This is often done with publishers who produce huge amounts of self-help books a year. These publishers pay you a flat fee to produce their self-help books. The books you write for work-for-hire publishers belong to the publisher you write them for and you hold no rights whatsoever beyond the fee they pay you per book. Many writers make a good living writing this way. You can find work-for-hire publishers by consulting a freelance writer's market guide online (see resources.)
Apply for an arts and entertainment grant that will pay you to write (see resources.) This is not the easiest route to go. The competition for grant money is fierce. You will need to write several letters stating who you are and why you believe the money should go to you. You'll need to outline your book project in detail. This is a time-consuming process that could result in nothing, so use it as a last resort, or if you believe your book project has such literary merit that it stands a good chance of attracting the people in charge of giving out grant money.