Green Lantern makes his second Silver Age appearance in Showcase #23 (Sept 1959). In it, a hint of a mystery is presented by Hal Jordan receiving a message from the Lantern to go check out a problem on the planet Venus. I like how the writers are seeding mysteries and not feeling the need to explain everything in these first couple issues. As a young reader, it must have been a pretty exciting reason to keep reading.
Eventually Oa, the Guardians of the Universe, and the Green Lantern Corps play a much bigger role in the Green Lantern mythos, but more time is given to the budding love “triangle” between Carol Ferris and Green Lantern/Hal Jordan in these first few stories.