Posted By:
CoreyRSh.anusThose cdv's Phil refers to were the components of a composite mammoth-plate photograph of an early 1870's (1872?) Boston National Association team. That photo almost certainly was the proof used to make a cdv of that team (by taking a photo of that mammoth plate). As such I really wouldn't call them a set in the sense that they were arguably unique cdv's made for a purpose other than distribution.
The earliest card set (issued over several years) in my view would be the Peck & Snyders, which were trade cards. At present there are six known, five team (1868 Lowells, 1868 Atlantics, 1869 Red Stockings, 1870 Athletics, 1870 Mutuals and 1870 White Stockings) and one player (c. mid-1860's Jim Creighton). These cards I think would be characterized as baseball cards by most collectors, and as such are arguably the earliest known set.
One other "set" that bears mentioning would be a recent find of the 1860's Haymakers (I forget the precise year but it was around 1867). Those cards all depict palyers from one team so in that sense they are part of a (team) set. But characterizing them as an early baseball card set then raises the question of how these cdvs were distributed. Were they distributed in such a way so to make them available to the general public and as such make them eligible to be characterized as baseball cards?
EDITED final time to restore my name to the post, which for some reason had been changed to anonymous by the previous edit.