Here’s the deal: Wikipedia is an encyclopedia to which anyone can contribute. That means it does not always have correct information. (It’s a lot like going up to the guy in the next cubicle who says he knows everything there is to know about Lady Gaga and asking him how much her meat dress cost. You might get the right answer, but you might not.) The site works hard to keep everything legit, but for authors who want to be taken seriously, that’s not enough.
All of the major publishers for whom I work, which number 8 or 10 these days, tell authors straight away that Wikipedia is not an acceptable source. Wikipedia is huge. It has information on seemingly every topic and can give a nice overview of a subject. It can also lead you to other, more reputable sources — just scroll to the bottom of the page and follow the links. But for major publishers, it cannot be used as a primary source, and you should adhere to this standard as well.
My husband insists that the number of errors found in Wikipedia articles is comparable to that found in Encyclopedia Britannica. Presumably he has a source for this claim — although he has never disclosed it. The problem, however, is as much about appearances as it is about facts. Wikipedia is not trusted, nor is it respected. If you want readers to take your work seriously, it is essential that you put in the time to use sources that are respected in your field.
What seems to go hand in hand with citing Wikipedia is excerpting Wikipedia at will. It is indeed a free encyclopedia. However, it is not free for the taking. That is, you cannot pull material from Wikipedia and pass it off as your own. You cannot pull huge amounts of text that someone else has written and provide only “Wikipedia” as the source. At some point this will be noticed and it will get you in trouble. I do believe many authors do not know they are infringing on copyright, but ignorance is not an excuse. Educate yourself and when you find out you’ve done it all wrong, do it again.
Now, research can be really, really boring. Keeping proper notations can be extremely tedious. And even when the topic is a passion, vetting online sources to ensure they are estimable is more work than many writers would choose to do, unless they were subjected to an outside force — e.g., a publisher, editor, or book coach, or perhaps an understanding of the market, that lets them know they are wasting their time if they take a shortcut to proper source notation. It comes down to this: With thorough research you look like an expert. With poor research you look like an amateur.
So be conscientious about your references. Investigate respected sources, and when you use someone else’s ideas, cite that person. Do this from the get-go. Do this thoroughly and without exception. Do not give in to the temptation to type your search term into Google, click on the first entry to appear (which usually is Wikipedia), and then copy and paste the material into your document. This is lazy, this is unprofessional, and this is plagiarism. And if that’s not enough, you will be hard pressed to sell a book that’s been prepared this way.
Wikipedia: friend or foe? It all depends on how you use it.