Other than things like spelling and punctuation, facts are pretty important.
If you want to include a topic in some way, or even a detail, it's important (in my mind any way) that it is represented as accurately as possible. The Internet is a great place for "learn enough to fake it" research; depending on the topic, the Internet is also occasionally good enough to in fact just "learn it", whatever it might be.
That's the writer's responsibility. The reader's responsibility is to keep in mind that a novel? Is fiction. The characters are not the author. Though if the reader is drawn in enough to believe that the author must have experienced described events or feelings in some way, I guess the writer has succeeded, yes?
As an example: many people might hesitate to include dark characters, depending on if they're a little uncomfortable with it themselves, or are worried about things like their grandmother reading the book. Nobody wants to answer awkward questions, sexual or otherwise, from their grandmother. I said, essentially, in "Where Am I To Go, Now That I've Gone Too Far?" (and points, if you know what song that's from), forget those people. I'm sure you love your grandmother, but forget her. She'll just be happy you wrote a book. Well, okay, she might be a bit scandalized. But, she's a grown up. You're, theoretically, a grown up. Grown ups write grown up things.
But where do you research some of these things? If you're inclined to go to Google and let your fingers do the walking, that's still all right. Even Wikipedia tends to be a good leaping-off point; you get a breakdown for "civilians", and then you can go to the articles and sources linked at the bottom of the page to get to the "real" source. There's also Google Scholar, which searches scholarly (and one might assume, professional) articles, books, web sites, and things of that nature, both in full text and abstract form. My public library's site, and I'm sure others, will also do various searches like this once you input your card number: Infotrac, Ebsco, the National Newspaper Index, and more.
Message boards, providing the posting members are truthful, may also be good material for your research. For an out-there example, if you have a character who is into "cutting" (self harm), there are message boards on which the members candidly (again, one assumes) discuss what they use, when they do it, and how it makes them feel. There are also message boards and blogs that are "Pro Ana", meaning have tips, tricks, and support for maintaining one's anorexia. These are two topics that I just "don't get" from a personal standpoint. I can't imagine starving myself, or cutting myself. With my psychology degree, I can understand from an intellectual standpoint, to a degree, what goes on in peoples' heads when they have these sorts of issues, but having what is ostensibly firsthand testimony is very helpful if you're trying to fictionalize an aspect of it.
Or, on a brighter note, say you are (for some reason) writing about a place you've never been to. A lot of states in the U.S. will send free tourism material to you upon request, and have a lot of that sort of material on their web sites. Wikipedia, again, can be useful here if you're looking for landmarks or activities. Fodor's is a company that writes travel guides worldwide, and their web site has a lot of information available on it as well. And of course, you can't go wrong with National Geographic, which has extensive articles on the web site, but also offers a data DVD set that includes every issue of the magazine, and is updatable when new material comes out.