The bigger a deal you make about it (suspicious packaging, nervous behavior), the more likely the customs folks are going to mistake your relatively harmless porn comic for something quite serious -- and they're going to search it.
I think the best strategy would be to wrap your comic up in gift-wrap and put a gift sticker on it that says "From: [your first name]" and "To: [somebody you know in America, preferably immediate family who aren't with you on this trip]". Then, put this gift-wrapped book in your suitcase. Make sure the wrapping is thin so that the suitcase checkers can tell it's a paperback book and not a box containing explosives or drugs or some such. It wouldn't hurt to have several books of similar size packaged in this way, porn or not. One draws attention, but multiple gifts seems perfectly reasonable for somebody traveling internationally. Also, thin wrapping paper's important to let dogs get the scent of the package. I don't know if they have book-sniffing dogs (lol) but they obviously have the drug-sniffing ones, and you'd rather the dog tell its master "this is 100% not a drug" than tell its master, "Well, I'm sort of on the fence with this one. Sort of hard to tell what it is, to be perfectly honest with you," forcing the human to open up the package to see what's inside.
If you'd prefer to place it in your carry-on bag, then do so but don't draw attention to yourself or to your bag. Be totally calm when they dump your bag's contents on the Customs table, if they even feel the need to do that. And when they ask what the gift-wrapped package is, tell them as very near to the truth as you can without disclosing the pornographic nature of the book. Tell them quite honestly that it's a rare Japanese comic book written by one of your little brother's favorite authors. Okay, maybe the "little brother" bit is a white lie, but how're they gonna prove that? This way, in a worst case scenario, if they rip open the packaging and see the dust jacket or flip through the book and see its contents, at least you won't have said anything to them which they could prove was a lie. It is, after all, a Japanese comic book. ^_^;