Here are two examples as to why Chessmen is Burroughs best work in this series.
 You can hardly conceive of a more ghastly creature than a spider-being who lives as a parasite on headless human bodies, but that is a perfect description of Ghek the Kaldane, one of the central figures of the book. Burroughs takes this repulsive monstrosity and makes him such a loveable character that you cannot help but like him.
 Burroughs not only wrote a good yarn, he wrapped his tale around a striking boardgame that he had invented – jetan, or Martian chess.
Its no real trick to invent a chess variant. There are thousands of them, and most of them are rubbish. What is so singular about jetan is that it is a good chess variant. I read Chessmen as a child, and after reading it, the first thing I had to do was make a jetan set and play the game. I whiled away several enjoyable hours with the game. John Gollon, a noted authority on chess variants, had a similar experience when he was writing Chess Variations. He thought hed include a chapter on jetan for some comic relief, so he made a jetan set and played a few games. He found jetan quite good – very playable and interesting. He then pronounced jetan not a mere novelty, but ... a respectable game.
These two singular achievments (Ghek and jetan) are not the only details that make Chessmen so enjoyable. Gahan of Gathol (aka Turan the Panthan) makes for a satisfying hero, and Tara of Helium fills the bill quite nicely for a damsel in distress.
The heros are noble, the villians are wicked, the cause is just, and the action is nonstop. Great escapist reading.