I remember that era very well. The early "serious" music knew what stereo was and used it well. Examples include Miles Davis' Kind of Blue, the Dave Brubeck albums, and all that great classical and light classical music on RCA Living Stereo, Mercury Living Presence, and Columbia Masterworks. Up until about 1967, stereo versions of albums cost at least $1 more than mono, which is equivalent to a $7 surcharge today.
When it came to pop music, much of it was recorded in 2-channel to keep instruments and voices separate for clarity, mixing, and partial retakes, but not with the idea of a left-to-right stereo sweep. When stereo started becoming the big thing, enthusiasts wanted to buy all their new records in stereo. So the pop labels simply took the 2-track masters and mastered stereo LPs with them. George Martin has gone on record that he recorded the early Beatles albums in 2-channel for mixing purposes and had no intention of releasing them as "stereo," describing the effect of voices and instruments at hard right and hard left as "ghastly."
It's also why many early collectible pop albums are worth more in the mono versions, because the hard right-hard left versions are distracting. Conversely, Kind of Blue and Mercury Living Presence are worth more in stereo because they were done right.
Some of the stereo small group jazz albums from this era are stunning, as they're recorded live in-studio with a palpable soundstage that positions the instruments in 3-D space. The stereo Kind of Blue and the Brubeck albums (all 6-eye Columbia) do this.
