Finding Blog Post Keywords

I asked in the BLB about using things like Publisher Rocket to see what keywords go with a specific subject. I was discouraged from that for blog posts. I am working on editing a Christian nonfiction book. However, my blog posts are not only about the topics of the book. How do I figure out what to use in blog posts to attract readers? I have not figured out the right words, so far as I know. I want to attract more readers. How do I improve the SEO of my blog posts. Do I need to do specific SEO for every blog post? Does WordPress support that? Thanks.

Ken