"letterhacking" is similar to "lochacking" for zines and shares some similarities with Mailing Comments in APAs: the writer is giving their feedback, in this case in comicbooks.

The trend brought us many legends in the letterwriting field, one of them the late and lamented T.M. Maple (Wikipedia mentions him here and Sequential has other links here). There was a time you could open any issue of a comic or comic news magazine and find a letter in it written by him; he's credited with over 3 thousand letters. Makes my current claim of 13 or 14 seem insignificant.

I gave him an invite to Comicopia many years ago. He had to turn me down due to lack of time available. He sent me a nice letter on his own letterhead. I haven't found it since my last move, which is a tragedy; he died from a heart attack a few months later.

Several of us in Comicopia have histories as letterhackers, and some of us still do (with me currently being our sole lochacker). With more and more changes to online discussion forums, as well as DC Comics abolishing letter columns a few years ago,the art of writing letters has been vanishing from the world. Hopefully those of us still doing it can slow that passing into history!