A lawyer is a good source. Even if there's a very well-maintained guide for laymen online, if it contains any errors you are in trouble. If a lawyer gives you advice that is wrong, you have some recourse.
I am not a lawyer, but my wife is an aspiring author and has spent a lot of time in discussion with some successful published authors. Here's what I've gleaned.
When you write a work, it is instantly copyrighted. You can sell these rights to others for various reasons; this is what the modern-day publishing industry revolves around.
You can self-publish and retain all rights. This means you have to find a way to print the books and pay for it all up-front. Bookstores won't carry it unless you convince them it will sell; you have to visit them and convince them to give you shelf space. If you want advertisement, you pay for it. If you want signings, you call the bookstores, make arrangements, and pay for travel. But when Hollywood comes knocking, you can try to negotiate full creative control by hiring a really good lawyer. Self-publishing is difficult if you don't have thousands of dollars of liquid capital.
Publishing handles a lot of this for you. The publisher pays for printing, talks to bookstores, advertises, makes cover art, edits, sets up reviews, arranges and pays for signings, and gives you a loan in the form of an advance. You get to write your book with money in your pocket, and don't have to worry about all the other things. Publishers take different degrees of ownership of rights to your works. You might negotiate a contract that doesn't make rights transferable; that means if Hollywood wants options they'll have to go through you. Or you might decide to let them have free reign. I've heard that in general, the more rights you reserve for yourself, the less money and help you get from a publisher. (Unless you're already a superstar author. Stephen King likely doesn't have to spend much time in negotiation.)
It works similarly with Hollywood. Books aren't written for the screen, and will always require some editing to work. If you want to retain full creative control, be prepared to have a much harder time landing a phat contract. If the studio doesn't think your work is marketable as-is, they're going to pass. Unless you're James Cameron or have similar proof that you can produce a marketable screenplay on your own they're going to get really nervous about giving you full control. Nervous execs pay less money or simply don't take deals.
Maybe you care more about the integrity of your work than profit. If that's the case, you'll probably have a much harder time going through publishers until you have proved your books sell themselves. Look at it this way: publishers and movie studios are investors. They analyze your work, decide how much money it will make, and offer you compensation for writing. If they balance the equation wrong, they can lose millions of dollars and decades of man-effort. Everything you do that makes them feel unsafe makes them see red numbers on the balance sheet.