I'm proud to announce that the science fiction short story "A Martian Odyssey" by Stanley G. Weinbaum (1934, following the version in the 1949 collection A Martian Odyssey and Others) is now available at Project Gutenberg.
It's revered as the first in a number of stories by Weinbaum that appeared in 1934-5 (followed by "Valley of Dreams", "Flight on Titan", "Parasite Planet", "The Lotus Eaters", and "The Mad Moon") that marked a milestone in the realistic depiction of alien creatures. He overcame the problem of making aliens seem like disguised humans, or being monstrous just to be scary; the aliens were meticulously logical (which was often extended to entire ecological systems), yet strange. (On the other hand, I find most of Weinbaum's non-alien stories to be decidedly lesser, and far more gimmicky and cheesy; aliens were as necessary for Weinbaum's fiction to shine as water was for Esther Williams.)
Gutenberg has a few other stories from A Martian Odyssey and Others, mostly non-alien stories, but including the sequel "Valley of Dreams", which picks up right where "A Martian Odyssey" leaves off, and manages to convincingly develop the nature of the alien creatures, and believably answers some questions raised in its predecessor, while leaving others unresolved.