Can I mention the words "Conan" and/or "Robert E. Howard" in the subtitle of my book? They are certainly well-known. Please note that they won't appear inside the book, only in the subtitle. Trying to sell books with your own barbarian hero is next to impossible. Can I use the above as keywords to draw attention to my own book in search engines?
Can I mention a character and/or name of a famous author in the subtitle of my book? [duplicate]
I think it depends on how you reference them. You could probably get away with something like "Inspired by the works of Robert E Howard" or "Inspired by the Conan the Barbarian series". Anything implying that your book features Conan the Barbarian and/or was written by Robert E Howard, however, is likely to get you in trouble - not necessarily for breach of copyright, but for false advertising.
I'm unsure whether this would actually work for SEO purposes, however. Searches for "Conan the Barbarian" are unlikely to bring up your book in the first few pages just because it has the words "Conan the Barbarian" in the subtitle, unless your book was already popular to begin with. Case in point: I've typed the words "Conan the Barbarian" six times in this answer, yet a Google search for "Conan the Barbarian book" does not bring up this post (at least, not in the first ten pages).
The problem you will hit there is trademark. Conan the Barbarian is a trademark and can not be used in commerce for competing products. (Just as UPS has trademarked the color brown, so you can't use it in the shipping business.)