Six Scorched Roses

No — it's a standalone novella with its own cast, so it can be read on its own. That said, it's set between the first two Nightborn books and adds background on characters who appear later, so most guides recommend reading The Serpent and the Wings of Night first.

It's numbered book 1.5 in the series — published after The Serpent and the Wings of Night (#1) and before The Ashes and the Star-Cursed King (#2).

No, it's a novella — a quick standalone read of under 200 pages in its original self-published edition, later reissued in hardcover by Bramble.