One of the first Warhammer books I bought was Nightbringer. That was a long time ago, but it stuck with me as enjoyable. Now, it's back in one of Black Library's most popular omnibuses that is back in print.
The Ultramarines are known for loyalty and courage, their martial prowess is legendary and is second only to the God-Emperor. There's a reason they are the most popular chapter to write about. Graham McNeill wrote six books in the Uriel Ventris Chronicles and column one is an epic trilogy of non-stop action, containing the novels Nightbringer, Warriors of Ultramar and Dead Sky, Black Sun.
What sets these books out from others is how easy they are to read for newcomers to Warhammer. There are no over-complicated words that you get in some Warhammer books - you know the kind, words where the author is giving words archaic/gothic or legendary qualities. I've read some books where it was hard to get through because it seemed the author wanted to cram as many of these "Warhammer words" into most sentences. It's okay when you know them, but can still be a slog. It's also not very friendly to newcomers, which is where I think the Uriel Ventris Chronicles shines. That's not to say it's simple, it's just easier to read and understand if you don't know the Warhammer lingo.
In typical Warhammer fashion is the brutality. It wouldn't be an overstatement to call this series brutal AF. The opening of Warriors of Ultramar sees Ventris ripping a Xeno's spine out through its mouth, which is described in detail. He really puts the "war" into Warhammer.
As for the stories - Nightbringer is a classic, and rightly so. You get to know Ventris, how he got to his position in command, his uncertainty of whether he should be and how he doesn't like breaking the Codex Astartes even if it means winning the battle. In book two, Warriors of Ultramar sees Uriel lead the team against a Xeno invasion with more questions raised over whether it's okay to occasionally break from the Codex Astartes. It also features the Ultramarines barbaric cousins, the Mortifactors Chapter who are wild and I need more of. And finally, there's Dead Sky, Black Sun where Ventris is on a Death Oath in the Eye of Terror for breaking the Codex Astartes and you get Iron Warriors and heretic Astartes.
Another part that makes this series compelling is that although it's the story of Uriel Ventris, the first three books could easily be referred to as The Uriel Ventris and Pasanius Lysane Chronicles, as Ventris has his faithful friend and Sergeant alongside him throughout. Even when he takes over as leader of the Deathwatch for a short period of time, Pasanius goes with him because where Uriel goes, Pasanius goes. It's a dynamic that works though as when either one of them questions their faith or commitment the other is there for them, giving advice or friendship to help them along.
There's not really much to dislike about the series - I'm struggling to think of anything! I think my only problem is that reading them so long afterwards, I lose a bit of dread at Uriel's fate. He gets sent on a Death Oath to die, but I know he doesn't because there are another three books left after this trilogy. Reading them at the time of release would have added more to it, but that's my own problem, not the books.
If you've not read the books before, then I highly recommend this. Nightbringer is a classic Warhammer story and Graham McNeill has a fantastic writing style. He pulls you into the gravitas that Uriel is meant to project to those around him and it's left me wanting to read the rest of the series. You don't have to be an Ultramarine fan (my favourites are the Dark Angels and Space Wolves) to enjoy this as it's everything you could want from a Warhammer series. Xenos, Daemons, heretic Astartes and questionable loyalist Space Marines from succession chapters. It has it all and I look forward to reading volume two.