As with most of the Planet Stories line, these are "forgotten classics" of the pulp era. Henry Kuttner created the character of Elak to fill the void left by Howard's death---his Conan stories were a huge hit with the readership of Weird Tales. C. L. Moore, otherwise famous for writing the character Jirel of Joiry, often touted as the first female sword & sorcery hero.
Northwest Smith, on the other hand, is a kind of Han Solo or Indiana Jones like character in a setting not unlike Leigh Brackett's solar system. All of the planets, or at least the inner planets, are able to support life, and there are native humans on Mars, Venus and elsewhere that in many ways are analogous to the relationships of Western countries in the nineteenth century with various indigenous peoples in their far-flung colonies.
Anyway, this will probably mean another delay in reading my Simon Green Hawk and Fisher books... unless I can get that going really fast and read a significant portion of the omnibus before these ILL's show up. So, in case you were really excited to see my thoughts on that (something I'm sure nobody was), well... sorry.
I'm also not quite sure how to review these yet; they're both short story collections, not novels. So I guess I'll have a go at it as best as I can.
One thing that's interesting, is that both of them supposedly sport a "Lovecraftian" vibe. Northwest Smith (the character in Northwest of Earth, naturally) is maybe like Han Solo or Indiana Jones, and the setting bears some resemblances to a Leigh Brackett or Edmond Hamilton solar system setting (common to sci-fi pulps of the time) but the antagonists are often things that you'd expect to see in a Lovecraft or Clark Ashton Smith story.
Anyway, I don't have any direct experience with either writer, so I'm excited to uncover some "forgotten classics" of the golden age of pulp.