The Second Ultramarines Omnibus collects 3 novels that deal with Uriel's return to Ultramar. Well, mostly Uriel. There are hundreds of characters and the most important characteristic of these characters is... their name, of course. Because you should already know about them. Because you're a Warhammer 40K fan and you should know everything. I am not, so I had no idea who they are and why they're so important.
The Killing Ground. One of the best novels, it starts out strong and it ends with a whimper. I noticed after reading a couple of Warhammer 40K novels that the less a novel deals with battle, the better it is. The Killing Ground deals with guilt and heresy and corruption, all very good themes. Oh, and there are monsters in this one, monsters that I'm not sure were the bad guys.
Courage and Honour. In this one, Ultramarines fight the tau, which apparently are the good guys in this Universe. They're the type that wants to help everyone live a good life and if you refuse their help, they will help you with or without your consent.
The Chapter's Due. The big battle between Ultramarines and Chaos Marines, led by Honsou (the protagonist of Iron Warriors omnibus). Everything is epic and for a book about full-blown war, there are plenty of moments to talk and discover characters. Unfortunately, the end is deus ex machina.
As I finished with the Ultramarines series (but I didn't read anything not collected in these 2 omnibuses), I have to list down the best parts in the series:
Warriors of Ultramar because it's about a zerg invasion with a pinch of gangsters trying to survive (extremely cheese, but I actually liked that part of the story);
Dead Sky Black Sun, because it was horribly gross, yet resembling comics trying too hard (I'm trying to avoid saying cheesy again);
The Killing Ground, because it explored some human themes;
The Beast of Calth, because it had the least amount of fighting, but the greatest amount of mystery. (It's part of the Iron Warriors omnibus, but I treat both series as part of one)