According to the SETI says blog: Originally published between April 14, 1940 and January 12, 1941, “Power Men of Mongo” picks up the storyline where the thirteenth installment, “The Ice Kingdom of Mongo” left off.
Artwork in these books is fantastic. Raymond is really flashy and the story from Harvey Flash Gordon was reprinted as the second Gordon Bread giveaway. I have three copies of #1 (two in great shape) and one copy of #2 which is a little beat up.
The complete story is available as a BLB or in many other reprints.