OMSFSM, Scrivener! I love it so much!
I wrote War of Ashes: Fate of Agaptus on LibreOffice and it worked well enough, but by the time the manuscript got to about the 50% mark, it started being a real chore to revise and restructure. By the time I finished, it was 221 pages that we were passing back and forth between writer and editor to handle in LibreOffice and MS Word, and it was rather unwieldy.
Towards the end of the process I downloaded Scrivener but I was too far along to try converting the document. However, when I worked on the Open Content materials from War of Ashes later on, I tested a variety of more advanced writing tools (mostly distraction-free editors and LaTeX-based power tools), with the most promising being LyX and Scrivener.
In the end I decided that Scrivener was a good option for me. It was powerful, flexible, inexpensive, multi-platform, easy to learn thanks to its great tutorials and manuals, and supported by a vibrant community.
When I say inexpensive… The macOS, Windows, and iOS versions go for $40, just enough to cover minor support costs, and you can download a free trial version. The Linux version (that’s the one I use) is free because it’s unofficial but the user community is very helpful, and I ended up sending my $40 as a donation because it was worth every penny.
It has so many features and so much flexibility to work the way you want. I use it right from the planning stage to create my structure and outline, and to gather my writing resources: publisher’s guidelines, references, examples, cheat sheets, lists of names, notes I jotted down, etc.
I use its metadata features to add notes to each section such as keywords, actions needed (“Write examples of play,”) references cited (“Top Secret, TSR, 1980,”) status (“first draft”), or who will be a collaborating author on this section.
I have Scrivener set up to save the draft in my working folder and create a backup on Dropbox, in addition to using iDrive for my regular computer backup. On top of that, every time I stop working for the night or reach a significant milestone, I compile an export version of my draft in .docx format and post it for my publishers on Google Drive so they can follow my progress, and have a work product in hand if anything happened to me.
But I kind of got used to its ease, and I forgot what an improvement it was! Except that when I stopped to take stock of my progress tonight, I looked at the page count and realized what a chore the current drafts would be to handle on a basic word processor. In addition, I had reworked several individual sections of the Tianxia Rules Companion this weekend and instead of being a major hassle to locate the sections to edit in the middle of a manuscript, it was a breeze. So I just had to say a word for useful software!