All great suggestions. I have just elected to go the agent route and he found me a non-traditional publisher very quickly. I did have a full manuscript, plus a book proposal.
If you just want to be sold online, then you don't need the traditional publisher. Their primary value is distribution to bookstores and their sales force that calls on the bookstores. Self-publishing is a good route.
Timing is also a big factor in deciding which route you will pursue.
Traditional publishers, once they accept you will take nearly 1 year to get your book in stores. Slightly shorter online. Only the famous get a sizeable advance or a signing bonus on their book.
Non-traditional publishers can have your book in print in 6 months and begin to get you in stores. Online is much faster.
And self-publishing is near immediate.
One thing that no one is mentioning here is that there is a cost, both with the online facilitator or non-traditional publisher. You have to commit to buy a certain number of your own books to market. You can expect to have to write a check when you sign the contract. Said more plainly, you pay to get published and if you don't sell your commitment, then you have a lot of wallpaper..... many of us still have boxes of our last book in storage. :(
No matter which publishing option you choose, you are the one marketing the book (unless you count press releases sent out by a publisher).
The successful authors either already have a great following from previous books or they have a large network on social media or in real life. They tap that. Or they hire a publicist or marketing person to do it for them.
It is a BIG commitment and I wholeheartedly agree that you need a professional editor. My agent had one, so that is who I used.
The last thing I will say is that you need to know why you are wanting to write a book. If you just want it as a calling card or a reinforcement of your expertise, then I would self-publish. You can still get physical books, but you won't have to make the big investment in inventory. I wanted to have my books in stores, but didn't want to wait a year, so the non-traditional publisher was a better deal for me. But I was prepared to make the investment.
I am combining pre-sales of my book (The Game Changer - an Allegorical Novel about Transformational Business Design) with the formation of a new network called thegamechanger.network. I also have a radio show of the same name. I will do a kickstarter campaign for the network, "giving away" a copy of the book to anyone who invests in the campaign at the base level (retail price of one book), plus they will get a membership in the network, which has over 300 radio shows available on demand. So I'm using the book also to build the new brand.
I am happy to answer any other questions since I'm on the other side of the decision that you are about to make.