There's a great plot analysis article (which can be found in multiple places, including
this one) on killer7 which you should definitely read AFTER you finish the game. It certainly helped me to get a better understanding of things.
Also, there's Suda's own Hand in killer7 book which provides some good additional background, as it includes an overarcing plot which he couldn't fit in the game.
It's in Japanese but English translations can be found online. (The article I mentioned above ties in well with this too.)
Other than that, I wouldn't be surprised if Dreamboum has another write-up up his sleeve at some point.
Not to mention discussing the game here, considering Kill The Past is pretty alive and kicking again - especially following the update Travis Strikes Again received shortly after launch. (Which also means No More Heroes 3 could well shed new light on certain killer7 elements, too.)