The Ultramarines Omnibus is a compilation of the three first stories of Ultramarines Captain Uriel Ventris and his friend and fellow Sargeant Pasanius.
Each book is a different story evolving the situations adn the state of the main characters but not much else.
The first book tells a dark about Imperial Guard, Dark Eldars and some... political problems. It is nice to grasp how mad this dread aliens are, and has interesting plot twists.
The second book is about a tyranid invasion on the planet Tarsis Ultra and the combined defense of Ultramarines, Imperial Guard and even the Inquisition and other elements I won't spoil.
For me the best of the three and almost the best WH40k book from the few I've read so far. You "live" the alien invasion each page.
The third and last book contained in the omnibus is a travel to the Eye of Terror (home of the Chaos forces, like another space, warped and twisted), so the main theme are Chaos Space Marines and demons. Not bad but the plot here was weak for my taste, and full of opportunism and tricks.
Overall, Ultramarines are the favourite chapter of Warhammer 40.000 of Games Workshop and many people, so this book gives a good introduction to them, with some good narratives and a story about Tyranids that for me makes the omnibus worth just because of it :)