Hi, Jon -
the best so far, up-to-date, and most accurate informations about to complicated story
and differences of/between the different editions of Cantos is offered by Peter Stoicheff,
"Hall of Mirrors - Ezra Pound and the End of the Cantos" (U of Michigan Press, 1995). If
I recall it correctly Stoicheff deals not only with all Canto editions in the US (New Directions) and
England (Faber); his only miss was the still different THIRTEENTH printing which came out
in the same year as his own book. The 13th, I think, is not only the most beautiful [size etc] but the
most surprising, too.
Although Barbara Eastman's book was very important and long overdue when it was published
at Orono, Stoicheff shows where a couple of her weanesses are. Electra. Take Eastman and
Stoicheff together, and I think you have all you need.