Now before any of you get too excited about this unusual find, the bad news: it's been dubbed into a foreign language, and this is the only version the poster could find. What the what?
This episode is listed as Season 1, Episode 20 and I initially thought that wasn't correct. According to IMDB (which does get things wrong sometimes), Episode 20 is "Up and Down Payments." "By The Book" isn't listed on IMDB, but it is listed on the AfterM*A*S*H Wiki page as Episode 20. Further proof that this is indeed from the first season comes from the appearance of John Chappell as Mike D'Angelo, who was removed between seasons 1 and 2.
In any case, Wiki lists the plot of this episode as "Mulcahy must stop a man who thinks he is Superman from endangering other patients", which certainly sounds like something I'd be interested in, aside from the whole M*A*S*H angle! But for now, this video will have to suffice.
One final thing: because I care about such things, it looks like the actor who is reading the Superman comics in question is the same guy who played the MP investigating the missing Mess Tent trays in the M*A*S*H Season 9 episode, "The Life You Save."
Nice catch Daniel, and thanks for the link!