As for ancient sources, I looked in Van Heck's Breviarium urbis Romae antiquae, which gathers ancient references to ancient monuments, etc. Of the 3 primary references to Trajan's Column, one is the text of the inscription on the base; and one from Epit. de Caes. (which I'm not familiar with) and one from Eutropius basically just mention the column as being above T.'s ashes. They're entirely uninterested in the relief.
inter divos relatus est solusque omnium intra Urbem sepultus est. ossa conlata in urnam auream in Foro, quood aedificavit, sub columna posita sunt. [Eutropius 8.5]
Richardson, A new topographical dictionary of ancient Rome, in the entry for Forum Traiani, has this to say about the relief:
The theory that the relief spiral had its origin as a book roll, a theory first propounded by Birt and endorsed repeatedly by others, ignores the excellence of the overall design, in which patterns are worked out vertically, as well as in sequence. And such continuous narrative is at least as old as the Odyssey Landscapes from the Esquiline, which seem hardly likely to have been derived from a book roll.
I had never considered the Odyssey Landscapes in relation to the Column of Trajan, but that's an interesting connection. I believe those paintings are from the 1st century BCE.
Coarelli, Rome and environs, appears to follow the theory of Birt that Richardson mentions. He says this (this is the translation by Jim Clauss and Dan Harmon from the University of Washington, Seattle):
The reliefs convey not so much a celebratory character as a documentary one. They have been interpreted as a figurative representation of Trajan's Commentarii, a prose narrative of the two Dacian wars that unfortunately has not survived.... This interpretation seems all the more plausible if we take into account the location of the column between the two libraries (Trajan's commentaries would have been kept in the Latin one) and the very appearance of the relief, which reproduces the form of an ancient book – a scroll (volumen).
This approach does indeed seem to view the relief as something akin to the graphic novel version of Moby Dick or the Bible, though without resorting to calling it a 'comic.'
Finally, one of the essays in Calvino, Collezione di sabbia (Collection of sand) is a very fine narrative reading of the relief – a result of Calvino's close-up inspection of the column itself, from the scaffolding that was erected around it at the time. It's been a while since I read the original Italian version, and I can't locate it on my shelves, but the English translation that I managed to find actually uses the word strip – once in 'scare quotes,' which may point to the double meaning 'continuous strip' (e.g., of parchment in a book roll) and 'comic strip' (and I believe the Italian word striscia, like the English word strip, can cover both these meanings).