With over 250 million books of kind in print, I think the 'dummies' books rate standing as a valuable resource for writers.
A good rule of thumb is to check out the credentials of the author(s). Does he have a stellar background in the field? Good enough to explain the topic to the rest of us? Look for information in 'About the Author' at the front of the book.
Next, skim the detailed Table of Contents. It's generally lengthy and perhaps even a bit intimidating. But if you're only looking for certain information, why plow through the entire book?
Before you read, familiarize yourself with the icons used to the left of the text throughout the books. (See Introduction) Be prepared for sidebars, which are chock full of facts, but don't let them distract you. Likewise, be aware of the appendices at the end of the book, as well as the index that follows. And, like any serious researcher, don't sit down to read without your notebook and pen. It's easier to jot down a fact (and a page number) than to go back later and try to find it.
I'm currently reading (parts of) Catholicism for Dummies. No, I'm not planning to convert, although the explanations I've read so far of its various theological tenets go right along with my own Protestant ones. (Yes, really--we're more alike than different, my friends.) My purpose is literary--the heroine of my cozy mystery series (currently in book four of six), Penelope Pembroke, is a practicing Catholic--which the reader needs to know in order to understand and appreciate Penelope as she goes about solving mysteries in the sleepy little town of Amaryllis AR.
What I need to know to make Penelope believable is right here within the pages of the 'dummies' book. (It also helps that I have a writing friend who is Catholic and can explain anything I don't quite understand.) While research ought to be verified with more than one credible source, these books are a good starting point for whatever you've a mind to delve into and import into your writing.
Have you used a 'dummies' book in your writing research? Which one--and what was your experience with it?