Note: The following reviews are fairly complete and so can and WILL contain spoilers.
John Altman's The Watchmen, despite making mention of it in the jacket blurb, barely touches on the subject of multiple personalities at all. The main character, Dr. Louis Finney is an ex-government scientist plagued by guilt over the various iatrogenic (therapist-caused) cases of MPD/DID he caused in the course of his work. The book seems to more or less follow a very "conservative" line- the Project BLUEBIRD could only induce dissociation in people who were already inclined to do so, and even then only through what amounted to torture via sleep deprivation, Thorazine, and other unethical methods. He speaks of "ruining" his patients, and the one who we are given fleeting glimpses of is shown as pitiful and is kept institutionalized long after she has "recovered"- though that admittedly could be motivated by a need to silence her.
That being said, it's not so much a strongly negative portrayal as an almost nonexistent one. Despite having overtones of MONARCH-conspiracy hysteria, the book manages to resist the urge to sensationalize its multiple personality subplot, if only by KEEPING it a subplot. Finney's past work is mentioned sparingly and never in much detail- the main focus of the book is his current decent into unethical measures to recover information from a terrorist. Most readers without a special interest in the subject will probably gloss over the few mentions we're given. The portrayal of multiplicity is definitely far from positive, but its impact pales in comparison to the book's portrayal of Muslims. In any case, I can hardly recommend this book.
Will Christopher Baer's book Penny Dreadful is something of a mixed bag- before finishing I was ready at several points to just dismiss it as being accurately titled and leave it. In the end, however, I found myself actually glad for having read it, even though it wasn't a particularly enjoyable experience. This is another book which isn't quite as multiplicity oriented as it first appears- or at least not in the way one might expect it to be. Despite the jacket blurb saying of its cast that "everyone... has multiple personalities", very few of them are 'classicly' multiple. It is far more accurate to say that they are all have what roleplayers call 'character bleed', or that they dissociate or compartmentalize their lives as part of the Game of Tongues that they play. Encouraged by drug-laced alcohol and shadowy manipulators in the background, the game they play is a barbaric one- they engage in 'battles' which end in biting off bits of each other's tongues, and the ultimate goal is to lose one's self/identity entirely, so that there is no longer any 'player', only the character.
Only Detective Moon and his 'alter ego' of Jimmy Sky appear to have identities fully independent of each other- and they're as dysfunctional as any of the other characters. They are both fully aware of the other's existence, though lacking in communication, and both experience lost time during switches. Each resents the time that the other spends in control of their body and both have plotted to have the other killed. Neither of them is a very sympathetic character.
What the book lacks in multiples, however, it makes up in medians- it was, in fact, reading this book that somehow helped me to finally make sense of exactly what the "Portrait of a Hydra" and other essays on the subject were trying to say. The median/midcontiuum presence in this novel outdoes its treatment of multiples in both quantity and quality. Many are as disreputable as Moon and Sky, such as the sadistic murderer Chrome/Christian and his simpering follower Mingus/Matthew, but a few are far more positive than one would expect from such a dark novel. The main character, Phineas Poe is hardly pleased with having "six or seven pretty concrete versions of myself knocking around in here" but it is this very ability to fluidly switch from one configuration to another which allows him to enter the Game and participate without losing himself entirely. Similarly, his girlfriend Eve's assumption of the identity of Goo, though mostly her undoing, ultimately allows her to escape from a dangerous situation and come to his rescue. It's still a disorienting and sometimes disturbing portrayal in both of their cases, but the same is true of the book in its entirety.
Finally, Dark Moon by David Gemmel is a book I can completely and wholeheartedly reccomend. If there were ever any book that I would unhesitatingly put on a multiplicity-related "required reading list", it would be this one. Unless you're absolutely averse to fantasy novels, I encourage everyone here to read it. Without trying to openly push an agenda, Gemmel handles his "split-personalitied" character better than any other author I've seen thus far- I almost wonder if he might have stumbled across some of the resources for healthy multiplicity that exist on the 'net and incorporated some of what he learned. The book toys with most of the classic media cliches and then throws them aside in favor of a more complicated worldview.
The two-person system in question appears at first glimpse to be a stereotypical good/evil split- a character with psychic abilities even views the main character Tarantio's 'brother' Dace as having the face of a demon. Nonetheless, no matter how many times others (and, more often, Dace himself!) call him heartless and evil, his actions towards his friends and his co-conscious brother show otherwise time and time again. Dace is more capable of positive emotion than he or anyone else gives him credit for. Also, despite originally splitting as a result of a trauma, they have long since healed from the wounds of their past and are some of the most sane members of the main cast.
The possibility of integrating comes up, but is ultimately rejected by both of the pair as an undesirable outcome. The two are a team and would hardly be separable even if it were possible- Tarantio may often wish that Dace would disappear permanently, but when he is faced with the prospect of that actually happening, he's lost and distraught. Dace's reckless fighting ability has earned them a reputation as the greatest swordsman in the land, Tarantio's more conservative temperament keeps Dace from getting in over his head, and their coexistence in the same body allows them to fend off a psychic assault that would have left anyone else fatally vulnerable. They are clearly stronger together than either would be alone. Finally, despite Tarantio's fears of being treated as possessed, the few times he finds a friend trustworthy enough to share the secret of Dace's existance, he finds acceptance, even from their lover. Ultimately, it doesn't matter if Tarantio is Dace, or if one is just a schism or manifestation of the other. They are who they are and for those who know them, that is enough.
Edit- cleaned up typos and added LJ-cuts for ease of reading. Sorry about that. ^^;;;;