When I start a new story, whether it's a short story, novella, novelette, or novel, I create three documents, without exception:
1) the story document
2) book description
Although most believe creating the story itself is the hardest part, what's equally difficult are creating the book description and proper keywords. Through trial and error, I find it easier to think of these as I'm writing the first draft than once everything is polished and ready for publication.
Full of questions, aren't we? No problem. One should always ask questions when they do not understand something.
In my former writer life of submitting queries to agents, I found (and I'm not alone) that the elevator pitch portion of the query letter, the 2-3 sentence summary, was difficult. How to summarize a 500-page novel into 3 sentences? It can be done. It just takes practice. Go onto Amazon and read book descriptions in the same genre as yours to get an idea on what to look for.
Keywords are the same way. At the bottom of each book's Amazon page are genres and sub-genres. There are courses by other authors who teach this. In the limited time here right now, I highly suggest going to Google and search it. You can even go to Nick Stephenson's Your First 10K Readers site to assist you with this as well.
Bottom line is that it's a good idea to start thinking of keywords and book descriptions while you're writing the book, because you can tweak them along the way.
Update on Discarded, Book 3 in the Central Division Series: the book is complete and the editing changes have been made. I'm going to do 1-2 polish reads, and then I'll hitting PUBLISH! For now, I am putting it through Amazon only, at least for the 90-day KDP Unlimited period, and then it will be published elsewhere.