So you've searched the Internet and found exactly the material that you needed. Now you can get back to writing, yes? No. Now you have to evaluate the reliability of your find.
First, consider how reliable our world is in general. Every newspaper and TV station in the US presents urban legends as fact more than once. Reporters for reputable newspapers and magazines confess to making up sources. Book publishers recall non-fiction books because the authors are exposed as frauds or they recommend diets that kill people. Con artists openly hawk their wares on the street, door-to-door, and by telephone. We are awash in a sea of deceit and manipulation outside the Internet, so why should Internet content be any better?
In fact, it's worse. Every crackpot in the world who owns a computer seems to have Web site. If you seek medical information, you'll have to wade through ten quack sites selling magical cures in order to find one trustworthy chunk of data. It's Gold Rush time in cyberspace and there are no laws.
Well, there are laws, but they're unenforceable. Let's say we want to set up a crooked operation of some sort on the Internet. When the police start to look for us, they'll first have go to our ISP (Internet Service Provider) to find out who we are. They'll demand that the ISP remove our illegal site and turn over our names so they can arrest us.
If we're only going to break Texas laws, let's set up our site on an ISP outside Texas' jurisdiction, like in Kansas. If we want to break US laws, we can set up our site on a Canadian ISP. If we're going to break international laws, we can set up some foreign spot where the red tape is thick and money talks. Distance is meaningless on the Internet and, as long as you have a credit card, you can buy space on an ISP anywhere in the world. A Texas warrant to access ISP records will carry very little weight in Iran or Hong Kong.
There are no absolutes on the Internet, you have to play the odds. Evaluate Web content based on what you know about the site's owners. The New York Times is more reliable than the National Inquirer. NASA is more reliable than the Flat Earth Society. Some sites are clearly fraudulent and easy to dismiss, but sometimes it's not so obvious and you may need to verify what you've found. If you notice that you're spending more time verifying your research than you are writing, it could be time to drop the subject altogether.
What difference does it make whether the information you use is correct? That bit of slander about a major political figure is just too juicy to pass up. That perpetual motion machine you read about looks like a great investment. That UFO site has proof that Queen Elizabeth is a flesh-eating lizard from the planet Woowoo, which is why she must be killed. What harm is there in telling your readers about your discoveries?
It's like this, the Internet may be wide-open, but the publishing world isn't. Once your work is in print you're open to lawsuits and criminal prosecution. If you can't trust it, don't use it.
Just because it's on the Internet and easy to download doesn't mean it's free. There are no orphans in cyberspace, everything belongs to someone.
Fair Use allows you to quote sections of someone else's work for purposes of critique or research, it doesn't mean that you can use their words to open your chapters or that you can use their graphics to illustrate your text. If you want to use someone else's work, most publishers will put the burden on you to get written permission from the copyright owners. Keep in mind that there's a lot of "borrowing" on the Internet and it's not always easy to locate copyright holders. The owner of the site where you found that great quote may have taken it from a book or another site without permission. As above, if you find yourself spending your days tracking down a copyright holder instead of writing, it may be time to consider an alternate. On the bright side, there's a incredibly large amount of public domain work on the Internet, and many people will gladly allow you to quote them so long as you ask first.
Paranoia is your friend when it comes to the Internet, but large infusions of rationality and common sense can get you through the worst parts.