I have a confession to make, I actually read The Fall of the Reach a decade before I ever picked up a Halo game. I really enjoyed the setting, themes, characters, and world-building which took what was already a surprisingly deep video game then expanded it into a truly rich setting. The fact the people at Bungie and later 343 Industries have chosen to let Eric Nylund's work become influences for the canon games shows their awareness of their IP as well as willingness to let all parts of it be important.
The Fall of Reach's premise is, essentially, the backstory of John-117, the Spartan program, and the Covenant War. We begin the book with the human race locked in a civil war between the Earth's government and Insurrectionists. Doctor Catherine Halsey believes she can solve this war with the creation of super-soldiers which will be able to do pinpoint infiltration and attacks that could end the war. Unfortunately, the process to create these transhumans requires children to be taken from their families and raised as soldiers from pre-adolescence. Worse, not all of them will survive.
It's a bold choice to start the story with a primary point of view character doing something so morally abhorent but Eric Nylund makes Doctor Halsey surprisingly sympathetic throughout. While treating the Spartans initially as nothing more than test subjects, she gradually comes to view them as beings akin to her children. Doctor Halsey's relationship with the Spartans would eventually be expanded upon in Halo 4, Halo: Spartan Ops, and Halo 5.
Co-starring in the book are William Keyes, future Pillar of Autumn Captain, and the Master Chief himself. William Keyes serves as a model Naval officer, divorced from the moral ambiguities of the Spartan Program and eager to make up for past failures. The Master Chief is an interesting mix of child and soldier, having been deprived of anything resembling a childhood but adapting by treating his missions like giant capture the flag sessions. Which is, given how Halo multiplayer became a lifestyle among gamers, kind of hilarious.
The war against the Insurrectionists doesn't last long, though, and the Earth soon finds itself encountering a hostile alien presence which seems obsessed with nothing less than the genocide of the human race. They destroy colony after colony with a ruthless abandon, washing over humanity's defenses as if they didn't even exist. The Covenant of the video games is often portrayed semi-humorously with the comical Grunts and their colorful attire but the one in the books is pure nightmare fuel. They are genocidal, horrific, and unstoppable with a technological edge that utterly dwarfs humanity's own.
The supporting characters like Chief Mendez, Cortana, Linda, and Ensign Lowell help enrich the storyline as the clock ticks down to the inevitable final confrontation with the Covenant at Reach. None of the characters but the main three are particularly well-developed but they aren't two-dimensional either and I developed an attachment to quite a few. Some of them, like Mendez, will go on to have big roles in the Expanded Universe as a whole.
As the title shows, there's no happy ending for this book and many of the Spartans we've come to care about will perish defending a planet which the Covenant will take no matter what. The book isn't exactly All Quiet on the Western Front but the tragic battle ultimately inspired Halo: Reach (reviewed here) and one of the series' most well-received titles. I will state for fans of Reach, though, that the book does not depict the battle of the game which retconned many of the events inside. The book shows a battle which lasted a few hours, maybe a day, while the game depicts a almost a week of fighting.
It should be noted the original book has since been updated since its initial release and contains several changes, additional content, and conversations to bring it more in line with existing Halo canon. I don't think these changes added very much since the book was already impressive as is. If you want an excellent piece of popcorn military fiction then you could do much worse than this book as well as the subsequent two volumes in the trilogy.