Another tool is yWriter.
This isn’t a tool for everyone, because it is research-first focused.
What I mean by that is that it’s a little clunky because background/research data is meant to go into it first, and then you are supposed to lean on that content to write your book second.
So for a non-fiction book, you would add all the data and facts and references, for a fiction book you would put in all of the important characters and plot points and things that the characters interact with.
This is so you always have a body of references to work off of so you don’t introduce inconsistencies.
Some people might find this software useful because assembling and fleshing out the underlying data is loads of fun and/or how they prep. Others might need this feature just to keep track of everything that goes into their book, as they might not be able to keep track of things like character quirks very easily in their head.
YMMV.