When thinking about the metadata for your book, don't skimp on detail: include all that is necessary - your book's title, subtitle, contributors, your own name, all the normal stuff.
Then we’re off into the realms of category and classification. Amazon gives me 7 keyword slots to fill in and I try to use every single one of them. Firstly I tagged my books as romance, then added historical romance and thought I’d done well; then discovered I should be more specific; not just “romance,” but "adult steampunk fantasy romance." (Not that I ever wrote anything to fit that classification!)
Instead of "historical" something more like "late Victorian Underworld zombie mash-up" seems to be the way to go.
If you don’t feel inspired, type a couple of keywords into Amazon and see what other authors use. Amazon even offers advice and recommends keywords for certain sub-categories. I was aware that BISAC Subject Codes existed – after all, I was a librarian! – but I’d never thought of using them in my self-publishing. BISAC Subject Headings categorize books by content so why not check them and see if their headings inspire you?
Think about including important story elements such as the occupation of your protagonist, the time period and place in which your book is set. Does it include any specific historic event? If so, work that in.
If you quote reviews in your book description, ask the reviewer if you may include a keyword. He may have written "Best thriller I have ever read!" but "best post-apocalyptic thriller I have ever read!" is even better.
Make excerpts from your book available where you can.
Provide links to your website and social media pages where allowed.
Add your author location if you want to appeal to your local audience.