I bet you haven't (SEO = Search Engine Optimization).
So far, I connected the optimization of keywords for search engines only with websites. For books SEO seemed rather pointless. But recently I learned something new. Through a free course I took (see below), I understood that online bookshops are basically search engines! And all platforms where you can upload eBooks have a field for keywords.
On Amazon for example, you're allowed seven phrases. Yes, that's right. You're not restricted to single words, you can use whole phrases. Other platforms might only allow single words, but often more than seven.
The problem with optimizing keywords is to find suitable words and phrases. They need to have little competition, high sales potential and must be used often by potential customers. That's no really easy. A hint are keywords that are suggested by the online bookshop when you start typing in the search bar. All results have to be evaluated, which is only possible on Amazon at the moment (only search for keywords in the Kindle-Shop, the other areas are of no interest and skew the results). Therefore I usually use the keywords I find on Amazon for all other platforms.
First, you need to determine, how many results you've got in total. The fewer the better. If you get more than 1000, the competition is too high. However, a keyword with only a few results isn't always good. To evaluate the worth of a keyword, you'll need to look at the results' sales ranks. If all sales ranks of the first page's results are high, the keyword is little used (few sales = bad rank). Are all ranks of the first page results in the Top 100, demand is high but the chance to get your book onto the first or second page of the search relut for this keywords are low. Finally, you should also look at the sales point of the results. If all the books with a good sales rank are sold for little money, it means that the potential customers are not prepared to pay more for this kind of book (that's the case in a few Romance categories). You need to develop a gut feeling for good values, but reaserching keywords is fun too. Just don't get sucked into buying too many books to read. :D
A short example:
Regen für Juma“ (Juma's Rain is coming in English soon too) I first tried the keyword „Steinzeit Afrika“ (Stone Age Africa). It had very few results. A closer look at the sales ranks showed that none of the eBooks had a rank at all. This made it very clear that no one really looked for this keyword. A search for „Götter Fantasy“ (gods fantasy) had a lot more hits, but the first 20 titles (Amazon displays 20 per search result page) had reasonable prices and sales I could reach. In addition, several of the results were exactly from the genre I wanted my book to be in (YA Fantasy). So, this keyword suited me fine.
A few additional tips:
A guy called Nick Stephenson offers free video training for this. it explains much better how to find good keywords. However, you'd need to leave your eMail-address with him, and he sends occasional marketing mails regarding his paid course. Since the information of his free offer already helped me, I'm currently trying his paid course. I'll keep you posted about it.
A tool that makes researchign keywords on Amazon easy is KindleSpy. It's reasonably priced and has a system, That shows you on a single glance with street light colors if a keyword is popular, has sales potential, and the amount of competition. It works for all Amazon-plattforms (like amazon.de, .com, .co.uk etc.).
With all SEO it is important to keep in mind that the keywords need to fit the book. If not, you'll lose readers who are disappointed that they didn't get what they searched for, or (worse) Amazon will close down the use of keywords (like they did with likes when those were abused).
And now, enjoy researching keywords. :D