Even more interesting than the visual framework though is the big exchange going on within it. I think fans justifiably mistrusted Jezebel Jet even before the big reveal at the end of Batman 680, so most of them/us probably interpreted her coaxing, armchair psychiatry - "This is a disturbed little boy's response to his parents' death," - as subversion, or prodding at best. "Don't let that bitch hustle you, Bruce! Batman never quits no matter how crazy shit gets!" or at least I thought. And I think Morrison anticipated this response from a good chunk of the fanbase.
But the funny thing is, if you find someone who never heard of Batman and show him this sequence, he'll say "This guy's nuts! He should listen to his wife," because Jezebel argues completely sound points and only our fondness for the Batman character averts our sympathies from them. Bruce reacts to her much as his most zealous fans would: "But this is how the Black Glove would work, isn't it? Wouldn't he even use you as a weapon?"
Excessively paranoid, no doubt, but Bruce is actually right! Following Jezebel's logical arguments would have played right into the grubby hands of the Black Glove. Only through unreasoning faith in the Batman persona can Bruce Wayne prevail over the forces of evil. Morrison is dabbling in absurdism again, rewarding Bruce's illogical choice to remain Batman in the face of a hundred better ways for someone of his stature to promote the good. Their whole argument takes on a religious flavor, the rational atheist Jezebel and the devout cultist Batman proselytizing to one another in the sacred temple of the Batcave. It all culminates when Bruce, hoping to convert Jezebel, brings her to his Holy Book, the Bat-Computer, in which all answers are contained: "If it's me, the Bat-Computer will know." But Batman can't read the Word. It's shrouded from him by static in a way that recalls another absurdist tale penned by Morrison, Animal Man's Coyote Gospel.
(Note: I skipped two pages after the first to avoid any sort of trouble)
In that story, the coyote is the prophet (obviously) and Animal Man is just an onlooker. In Batman though, Morrison flips the scene, so the scripture is lost on the prophet but clear to the onlooker. There's good reason for the reversal. In The Coyote Gospel, Animal Man can't decipher the document because it details a decree by God Himself for an endless cycle of torment. While the coyote has come to terms with the violent way of the world, living that violence endlessly everyday, the pacifistic Animal Man cannot face it. In Batman in the Underworld, Batman can't decipher the display because it depicts a part of his insane history that he's willfully surpressed. While Jezebel freely acknowledges the absurdity of Batman's adventures, emblematized by "Zur En Arrh," Bruce cannot face that symbol of his contradictory past. Both "Man"s can't swallow those truths that defy their respective systems of belief.
Notice that for all the talk of black casebooks, Bruce never actually reads them. Alfred "left them in his desk" or borrowed them to "transfer their lurid contents to memory stick." Alfred is right to keep them from Bruce. As we see in this issue, his mind can't take the strain.
Page 1: The gargoyle "henches" (it should be a word) for Le Bossu. His blade, as we learn on page 7, is coated with a latently-acting drug which makes Batman susceptible to the trigger phrase later in the issue. I like that the gargoyle wears sneakers; he refuses to sacrifice practicality for flamboyancy.
Page 3: The "old movie" alludes to The Black Glove film, whose poster we first spotted in the Club of Heroes arc.
Page 4: "I believe I committed most of your handwritten notes to bat-computer files." He began this undertaking in 664
"Innocent lovers corrupted and destroyed." John Mayhew and the Black Glove framed (it's implied) his wife's lover, Mangrove Pierce, for her murder.
Page 5: I believe this is the first time Batman mentions a connection between his parents and the cast of The Black Glove, which seems sloppy on Morrison's part considering we met John Mayhew and the Glove way back in 667. Also, Mangrove Pierce changed his name from the previous page to "Mongrove" Pierce, which he thought sounded less gay.
The monitors display (probably) the faces of Marsha Lamarr, Mayhew's murdered wife and co-star of The Black Glove, "Mongrove" Pierce, and Mayhew himself.
Page 6: "Nothing less than the utter ruination of a noble human spirit." The Devil wouldn't settle for less.
Page 9: "It was like I'd known you all my life. It felt like meeting you was always meant to happen." Batman doesn't spout romantic drivel like this. The Black Glove has cued his mind to attach to Jezebel.
Page 10: "Ed Sheldrake" is possibly related to Cyril Sheldrake the Knight? Maybe the Black Glove is trying to advance the illusion that all of Bruce Wayne's friends have turned on him. Seems tenuous though.
"Schizophrenic?" On this page, Morrison lines his red herrings in a row - Thomas Wayne, Bruce Wayne himself, Alfred - before shooting them down over the course of RIP.
Page 16: "You know like the dead man's hand the Joker dealt me when I went to Arkham Asylum." DC Universe 0, as if you forgot! -- Crustaceous Cass
Page 19: The monitors display the leering golem from Robin Dies at Dawn, the one Bruce hallucinated in Batman 672.