This article from Smithsonian dives into some of the history of the S’more. The focus seems to be more on the individual components than the actual S’more, but it is very interesting.
The oldest ingredient in the s’more’s holy trinity is the marshmallow, a sweet that gets its name from a plant called, appropriately enough, the marsh mallow. Marsh mallow, or Althea officinalis, is a plant indigenous to Eurasia and Northern Africa. For thousands of years, the root sap was boiled, strained and sweetened to cure sore throats or simply be eaten as a treat.
As for how the graham cracker became a part of the s’more, the snack’s true origin remains unclear.
The first mention of this treat is in a 1927 edition of the Girl Scout manual Tramping and Trailing with the Girl Scouts. In a nod to the treat’s addictive qualities, it was dubbed “Some More.”
The term s’more is first found the 1938 guide Recreational Programs for Summer Camps, by William Henry Gibson. Some think the s’more may be a homemade version of the Mallomar or the moon pie, two snacks introduced in the 1910s.