I got my copy on friday - had a busy weekend, so not had a chance to absorb it in full yet, but I've skim read it. Mithras is spot on, it's good.
It doesn't really answer any questions regarding the nature of the Tyrant Star. It's also fairly open ended: the intention seems to be to let GMs "fill in the blanks" regarding the Tyrant Star and the outcomes of decisions made by the players.
Like the rest of the series, it provides a selection of memorable vignettes and scenes, with some solid background setting. Mara, for example, is the closest we've seen in 40k RP to an old fashioned "dungeon crawl" but in an excellent and well developed grimdark horror-packed setting. It's also designed to heavily favour player casualties, with plenty of alternate PCs waiting in the wings when beloved characters die in horrible ways.
Just me wishing aloud, I guess.