Microsoft Word is probably the best choice. You can crank up the grammar and spell check options, and it will do a good job of diagnosing technical problems. (My decision to follow Canadian spelling does cause some grief.) You can find people willing to proofread (copy-edit) online, and in some cases, running the document through Word's grammar checker is all they do.
Importantly, most of the distribution platforms support importing straight from Word.
Fiction versus Non-fictionThe big bucks are in fiction, but only a tiny number of authors get that money. Book sales follow a power law (or whatever they call it) -- the top tail captures most of the distribution. This is true for all genres of writing.
The advantage of non-fiction is that you can show up in customer searches. For example, there are very few books on breakeven inflation or inflation-linked bonds, which is one reason I wrote that book early in my sequence of planned books.
In fiction, people will not find you in searches. For example, if you invent the fictional world of Snurffleduffle, people are unlikely to search for "Snurfflefuffle", at least until your book is famous. The problem is achieving that breakout to fame.
Meanwhile, try to remember your last visit to the bookstore (since you should not be visiting them now!). Fiction shelves are lined with famous, dead authors -- and you somehow need to steal their shelf space. (My understanding is that the easiest place to break into those bookshelves is bodice ripper fiction -- the consumers of those books like variety, and new stuff.) Whereas for non-fiction, books that are too old are less attractive than a newer one (other than recognised classics).
Traditional Publisher versus Self-PublishedThe economics of publishing has been a disaster for some time, and publishers have largely removed support for authors who are not already well-known. If you are a nobody (like me), a publisher will get you into bookstores, but in order to get anywhere, you have to self-publicise ("build a platform"). (Since I am writing highly specialised books, I saw no point in going to a publisher.)
Self-Publishing PlatformsI work with Amazon's Kindle platform, and Draft2Digital. Both cover e-books and print-on-demand paperback.
There are "vanity" publishers, but most of them had the old model of doing a print run of 1000 books (or whatever) that you end up stuck with in your garage. Might be easier to work with, but I have heard very little good about them.
You could try selling e-books yourself on a website, but the sales platforms I saw offered zero support for dealing with things like VAT. You also lose the potential for organic sales on the platform, which are also trusted e-commerce sites.
To what extent possible, you want to have your own ISBN. The ISBN owner is the publisher of record of the book. The distributors offer free ISBN's, but they become the publishers of record. If your book does not sell, that might not matter, but things could get complicated if the book is valuable.
FormattingAn e-book chapter is a web page that is being rendered on a specialised browser (the e-book reader). Formatting is just a question of using styles. For fiction, you can just upload a Word document to the distributor, and it will probably look very good. (The Draft2Digital styles looked very slick.)
Problems appear if you have any sort of graphics or tables, or things like footnotes. These specialised features forced me to do my own formatting work.
Amazon and Draft2Digital have a conversion option to also create a paperback edition. Once again, if you only have text, these are probably good enough. However, since I am working with graphics, have extensive sub-headings, and have footnotes, there are a lot of potential problems with page layout. Publishing is an art, and people will instinctively note problems with poorly formatted printed editions.
(c) Brian Romanchuk 2020