I have mentioned more than once that my newest book, Unlikely Prophet, took over thirteen years to write. (That’s based on the oldest snips of story I can find on my hard drive.)
I have had several people as me, “What took so long?”
Part of it was research. Although it doesn’t show in the book (I hope, if I’ve done my job well), I had to do quite a bit of research on all different topics, and even having My Very Own Physicist™ to help in some areas, I still had a myriad of others to worry about and talked to so many different people to get details right.
Also, I was working on the main books of the series, and wanted them out (or at least some of them; I managed two before caving) before publishing the prequel.
Then – my husband got sick. Through three and a half years of caregiving, and then his death, it was harder to write although I slogged on. But this book was more difficult than the others (for reasons you’ll know if you read it). I finally felt I could concentrate on it and finished it this last year. I’m very proud of it, and have dedicated it to the memory of my husband, Stephen King.