I can think of two examples of opportunities for direct experience that came my way after the book was written -- written, published, and on the library shelves. The first was my trip to Antarctica, which I made well after my two books about Shackleton's Endurance adventure were in print. (Shipwreck at the Bottom of the World, and Spirit of Endurance both published by Crown.) Of all the direct experience of Antarctica that I had, the one that I wished I had known before writing about it was....... silence. I have never experienced such profound silence as I encountered in Antarctica. I remember clearly the day I wandered through an expanse of volcanic debris on Ross Island, with no sound at all but the crunching of grit under my feet. No animal sounds, no wind through trees. No mechanical noises. Yes, there are other places in the world to experience deep silence; that is true. But without a trip to Antarctica while I was writing about it, I overlooked that silence as a meaningful part of the story.
The second example is from a class in wet-plate collodion photography that I was able to take after I wrote a book about Civil War photographs (Photo by Brady: A Picture of the Civil War, published by Atheneum.) During the three-day workshop I was relieved to discover that I had, in fact, described the process accurately. Whew! But one little thing had escaped my researches at the time, and I think it is because it was so commonplace for wet-plate photographers that none of them had mentioned it. The silver nitrate solution, which is what turns jet black when exposed to light, thus creating the image on filmed glass, gets on the photographer, too! It's unavoidable, especially in rough, outdoor conditions. In the darkroom the clear silver nitrate might splash on the photographer's hands or clothes; the moment that photographer steps out into the light, those splashes turn black! I left that three-day workshop stained everywhere -- on my feet (I'd worn flip-flops), on my hands, on my jeans, even a smudge on my cheek where I'd scratched an itch with a silver-nitrate-dipped finger. And it didn't wash off, it just had to wear off after several days. There is no way a field operator in the Civil War could have avoided tell-tale drips, drops, and splashes of black on his skin and clothes. I wish I had known that when I was writing.
Sometimes this kind of direct experience is not available to the researcher. But this is why my recommendation is to experience as many things as possible. Some day you may be writing about that subject and you'll remember the silence and the stains. These are the things you won't find in the library stacks.