I've read all four books in this series, including the three novellas. I rated everything at 3 stars, except for Kerry-Anne, who got 4. This one gets 2, and it's indicative of what I feel is bad execution of a strong premise.
The premise of the series is those who choose to end their lives are given the opportunity to use their perspective to help the still-living avoid a similar fate. While the alternative is permanent purgatory, versus an automatic "move-onward" for the general population, it's not a balanced choice but is an intriguing one. The Station is similar enough in earth appearance to be recognizable, and the lack of time measurement adds just enough lost equilibrium to make the storyline satisfyingly disconnected from our world. (While I did appreciate in this book that Piper was attempting to track time by associating events with case history, there was an odd reference to "staff quarters" that didn't work well. There are no days and nights or downtime.)
The first novel involves Piper volunteering twice, once with Sloan, which sets up a major plot line, and then with Abby, who is instantly both forgotten and literally dropped. The second novel introduces the Jess/Cole volunteering trip, which is Piper's final effort before the story goes off the rails. She volunteers three times, resulting in one suicide. That's it. While I can appreciate the series was not set up to be "Touched by an Angel", nor would I have enjoyed it if it exclusively had, the shift in tone and plot was jarring.
Instead of being fed morsels as to how the Station functions, it's just aliens. They were bored, and tall, so they created the stations. They live on a planet very much like ours, aren't very alien-like in appearance, but they're Seers and wouldn't you know it, but Piper is a Mary Sue now (and alive! again?!) who is so bogged down by added-on plot that it detonates her relationships and character. I can't understand why a dead girl finding her place in the afterlife wasn't enough. Now she has to be a telepathic seer in a love triangle with an alien? Her relationship with Sloan can't function because she's a Mary Sue. She can't volunteer for the same reason. Despite arriving at the same time as Kerry-Anne (and having significantly fewer notches than Annabelle, whose plot line is dropped like a rock to fit in said love triangle), Piper is chosen for upgrade to mentor and admittedly is terrible at it. She spends much of the book running from place to place and sobbing everywhere.
In addition to the Mary Sue weight, Rush killed all initial story momentum. He treats Piper like a small child and she seems to love every minute of it this time around. He betrays her confidence and tells Sarg about Ryan Burke and she instantly forgets the moment she sees him. Prior to his introduction, we had a story about a cluster of characters, two of whom volunteered for two of the others but who were not able to save them. There was a fascinating potential there, but it was dropped in favor of Rush's alien gobbledegook. Book 2 opened the possibility of multiple characters participating in the same volunteer mission, without being able to directly communicate or identify each other, and that potential was ignored. Annabelle's story opened the doorway to a potential story involving a character being marooned on assignment. It was ignored. (I wasn't sure why it was referenced that the volunteer had to be in transit to their assignment, and not just at work.) We randomly got a plot line involving Carlson suddenly becoming a sexual abuser. I can't recall his life history (update: It was a focus of the Mallory vignette), but it seemed out of left field to force Piper off the badly managed station. (Why was our station running so below capacity but Ryan's station needed a new wing to keep up with demand?)
The wonderful throwaway from the last novel that at least 70 years have passed for Piper and the time disconnect - you can arrive after someone despite dying first - led to more potential for encountering characters from Piper's and the others personal lives. Her ability to see across the bridge left the possibility open that she might interact with her lost best friend, or others who proceeded directly across. That's not a dig. That wasn't the story here, but I feel there was more to do without going the alien route. I feel like sticking to the simple but inspired plot would have led to a much stronger story.
I wouldn't care to hear from Piper again, but if Trish Dawson decides to write more from Kerry-Anne or Ryan Burke's perspective, I'd be the first to buy it. There's more room for growth in this universe, but this is one of those cases where I wish the story I was reading was from the perspective of anybody else.