In such cases, it's often best to let the original author speak for themselves. Fortunately, Doug Schwegman does so at the start of his article where he introduces them to D&D.
. . . I believe it is a logical addition to the D & D scene and the one I have composed is a hodgepodge of at least three different kinds, the norse ‘skald’, the celtic ‘bard’, and the southern european ‘minstrel’. The skalds were often old warriors who were a kind of self appointed historian whose duty was to record the ancient battles, blood feuds, and deeds of exceptional prowess by setting them to verse much like the ancient Greek poets did. Tolkien, a great Nordic scholar, copied this style several times in the Lord of the Rings trilogy (for example Bilbo’s chant of Earendil the Mariner). The Celts, especially in Britain, had a much more organized structure in which the post of Barbs as official historians fell somewhere between the Gwelfili or public recorders and the Druids who were the judges as well as spiritual leaders. In the Celtic system Bards were trained by the Druids for a period of almost twenty years before they assumed their duties, among which was to follow the heroes into battle to provide an accurate account of their deeds, as well as to act as trusted intermediaries to settle hostilities among opposing tribes. By far the most common conception of a Bard is as a minstrel who entertained to courts of princes and kings in France, Italy and parts of Germany in the latter middle ages. Such a character was not as trust worthy as the Celtic or Nordic Bards and could be compared to a combination Thief-Illusionist. These characters were called Jongleurs by the French, from which the corrupt term juggler and court jester are remembered today . . .
I wanted to put the Bard into perspective so that his multitudinous abilities
in Dungeons and Drageons can be explained. I have fashioned the character
more after the Celtic and Norse types than anything else, thus he is a character
who resembles a fighter more than anything else, but who knows something about
the mysterious forces of magic and is well adept with his hands, etc.
(Schwegman, Doug. Statistics Regarding Classes: (Additions) — BARDS. Strategic Review Vol 2, No. 1, February 1976, p. 11.)
To recap with clarity -- a mixture of:
- Norse Skald
- Celtic Bard
- Souther European Minstrel & Jongleur
- Tolkienian historian
Mr. Schwegman gives no list of references aside from this. The incredible prequisites (mid-level fighter and mid level thief) of the Gygax style of Bard strongly remind me of the 20 years of training of the Celtic Bards; a change made by E. Gary Gygax in the version in the AD&D Player's Handbook.
Edit: as a side note, the initial presentation was as a standalone class; Spells and thief abilities were those of a member of those classes of half the bard's level. In AD&D, Gygax revised them, and this was changed, and the prerequisites of Fighter 5-8 level then thief of 4-6th level then finally to bard were added, and give more of a celtic feel.