Or two stand-alones and a trilogy?
That comes closest. Although in the strictest sense, the word "trilogy" should only be used for three separate, complete individual stories that collectively form a larger arc or continuity, rather than a single story split into three volumes. So it could be argued that the five books in all form a trilogy, the third part of which was published in three installments. But that interpretation is unlikely to catch on. Maybe they should just be called a "series," though it's kind of a loose series.
I was just curious if there are any other novels that should be read in conjunction with these or if these five pretty much stand alone.
I'd say
Forge and
Heart are pretty much self-contained.
Forge is largely a story about Spock's adolescence and how he came to decide to join Starfleet, with a frame story of Captain Spock (after Kirk's apparent death in the GEN prologue) being reunited with a character who played a crucial role in those events. It introduces a Romulan character who will figure in all five books (and a short story by the same authors in
Tales of the Dominion War), but otherwise its ties to the other books -- or indeed any other books -- are minor.
Heart is set decades later and fills in the backstory about Spock's marriage as referenced in TNG: "Sarek," as well as dealing with the Battle of Narendra III from "Yesterday's Enterprise" and featuring Picard and the
Stargazer. It also establishes a lot about Romulan politics in what would later be called the Lost Era, and what it established about that has been built on in later books, notably
The Lost Era: Serpents Among the Ruins.
Soul is set decades after that, in the wake of the Dominion War, and involves a much older Ambassador Spock pursuing his Unification mission, as well as flashing back to the proto-Romulans' schism and departure from Vulcan, basically retelling the story of the Sundering that Diane Duane had earlier dealt with in
The Romulan Way and
Spock's World. The first book tells Surak's story on Vulcan rather differently than Duane did, but the second book's chronicle of the interstellar migration follows Duane's version very closely while expanding on it. There was a change in editors between books 1 and 2, as I recall, and I've always suspected that it led to a change in attitude about whether to acknowledge Duane's work, though that's just speculation. Anyway, the "present-day" story revisits the Romulan characters and political arcs established in the previous books. Despite the "
Vulcan's" in the titles, it's the Romulan elements that provide the throughline for the series.
