I've just completed Parallel Worlds by Michio Kaku in preparation for my second novel. I love researching a topic that I find intriguing, jotting down notes as I go, finding inspiration by a particular fact or theory. I haven't stated anything earth-shattering here; research for fact-based fiction is common sense. While some research is always good, too much is always bad. Here are two examples.
I have a shelf at home that contains some of the physical books I ordered to prepare for Lifeless. These are in addition to the digital books, articles, and research papers I had studied before completing my first draft. Several of these physical books took their sweet time in transit - they were rare, out of print, or the shipping was just slow. In either case, I no longer needed them when they miraculously appeared on my doorstep. I was satisfied with the work I had completed on those topics. Now there they sit, with their facts, silently judging.
I keep thinking of taking them up, tweaking some things in the book. But I know me - I'd look at the cited works in the back and I'd find other handy materials. Just two more books, then I'll be done, I'd think. But those books would have interesting citations. And thus I would fall down the rabbit hole, my orange highlighter tumbling in after me.
There is another problem when researching a fiction book too deeply. Facts are not rigid and interpretations are endless. Sure, you've read the most popular research, but you know the historical dubiousness of popular research! Then of course you could talk to experts, but experts often disagree with one another. Who do you trust? If you wait long enough, new discoveries will be made that reveal the inaccuracy of 30% of your reference material. So you fix those things; you keep thinking of that expert in your audience, given the book by a well-meaning friend ("look at this, Betty! I saw 'rocket engineer' on the back and thought of you!"). You don't want to disappoint that expert.
My greatest challenge, and I know I'm not alone here, is accepting that you will disappoint that expert. If you don't disappoint that expert, you'll disappoint another one. You can't win over everyone. Someone will call you a lazy writer who didn't do enough research. That's OK. Your goal as a fiction writer is to create a coherent world for your characters to inhabit, to suspend disbelief, and to move on to something else when you are finished. Out-of-control research can prevent that from happening.
And when all else fails, be vague. :)