Between is the place is beween the mortal world and the place where spirits go in the Afterlife. First mentioned in Ghost Story.
Between is where a humans spirit goes when there is some sort of irregularity with their death. And maybe for other reasons not mentioned. Harry Dresden asked if it was Purgatory and Captain Jack Murphy answered that he wasn't there to "cleanse himself" but he could call it that if it worked for him.
The work Uriel does is "what I ever have done". Uriel explained that he and his colleagues, like Collin Murphy and Ron Carmichael, labor to ensure freedom of choice, of will. It is his duty to "protect and nourish" the freedom to choose between good and evil or the distinction is meaningless.
The trains take souls to What Comes Next. Choosing to take a train to What Comes Next is an act of faith.
A southbound train heads to somewhere like Hell, one almost hits Harry Dresden when he arrives. According to Ron Carmichael, the southbound trains run pretty quick lately and that Dresden probably didn't want to get hooked up with one. Captain Jack told Dresden that with his record, he could just as easily find himself on the Southbound Train as talking to Saint Peter.
Some Things there that are worse than death, that have ears everywhere and that are not safe to talk about openly.
What comes next
- Uriel informs Harry Dresden that What Comes Next involves things like: forever, eternity, judgment. What Comes Next means accepting "the consequences for all that you have done while alive. When judged, what you have done will be taken into account. Your fate, ultimately, will be determined by your actions in life. You can not escape the consequences of your choices."
- Captain Jack Murphy informs Dresden that "What comes after isn't for people who are rubbernecking over their shoulders or bitching about how unfair they had it." He also states that they have to sort out how Dresden got screwed over and then he can move on to "what's next". Later, Dresden paraphrases for Karrin Murphy what Jack said (omitting who said it): "What came next wasn't for whiners or rubberneckers." Murphy comments that it sounded like something her father would say.
- According to Uriel, Collin Murphy, like the others who work for him in his Between Office, is there because he's not ready to "what comes next". When he's ready, he'll take the next step.
In the series
In Ghost Story, after Dresden is shot and died, he finds himself in Between. Ron Carmichael's ghost saves him from a train about to run him over and escorts him in his Mustang to the Office. There, Dresden uses his Sight seeing the presence of an Amitiel, near the elevator, who is disguised as a guard and whose gun is really a sword.
Captain Jack informs Dresden that there's an irregularity with his death, that the "opposition" wants him out of the way, breaking the law—they cheated. Jack assigns to him the task of solving his own murder in order to save three people he cares about from danger, without telling him which three. It's risky because Dresden could be trapped there as a spirit for eternity if he never finds the killer.
When Dresden asks what happens now, Uriel informs him that it's up to him, that he can always work for him in Between: it's challenging and he can find an use for his talents. When it is time for Dresden to choose, Uriel takes him to Chicago Between, stopping in front of two doors, one leading to the Office where Collin Murphy and Carmichael work, the other What Comes Next.