As the first book set in the Halo universe, Eric Nylund's Halo: The Fall of Reach describes the training and induction of the first-ever Spartans, including John-117, the Master Chief. It follows the Spartans through some of their earliest missions, depicts humanity's first contact with the Covenant, and culminates in the battle for planet Reach, a human colony. I don't often read fiction, and I am not sure I've ever before read "expanded universe" fiction for a game or a movie, so Halo: The Fall of Reach was a bit of an unusual read for me. However, since I really enjoy the Halo games, universe, and characters, and since this book came highly recommended by other fans of the series, I figured it couldn't hurt to give it a try.
Unfortunately, per my review policy, I can't actually review this book: the Halo series, and indeed the copyright to the book, is owned by Microsoft, by whom I am employed by. Instead, I want to highlight a few things that I found interesting in the book. Be warned that this post will contain some spoilers for the book and other parts of the Halo series.
I've never been involved in professional game development, but my understanding is that game mechanics and environments are typically built first, with story added later in the development process; this necessarily means that the plot is shoehorned into a set of scenarios and environments that might not actually be an optimal fit for it. This in turn is one of the reasons video game writing is notoriously mediocre. To the extent that this is true, it is especially true of the first game in a new franchise. However, Halo: The Fall of Reach was actually originally published on October 30th, 2001, about two weeks before Halo: Combat Evolved was released. This shows that Bungie had an impressively thorough understanding of the universe they were building at that relatively early stage in the franchise's life.
On the other hand, much of that universe might still have been conceived in a short amount of time. In an afterword to the Halo: The Fall of Reach, Frank O'Connor writes that "[t]he book went from conception to final print in a staggering four months." That is certainly impressive for 352-page book!
According to the book, Spartans in MJOLNIR armor are able to sprint at superhuman speeds: "The Chief and his team sprinted up the half-kilometer sandstone slope in thirty-two seconds flat." That is a speed of 35 miles per hour (56.2km/h) on an upward slope. This is somewhat at odds with the relatively sluggish pace of movement in the first three Halo games. However, sprinting was fairly unpopular when it was added to the game series, as many people felt that it changed multiplayer balance for the worse. (I don't personally have an opinion on that, but obviously, gameplay balance is more important than being accurate to lore.)
On the subject of gameplay being at odds with lore, the game Halo: Reach also depicts the fall of Reach, but from the perspective of a group of Spartans on the ground rather than from the Master Chief's perspective. It also infamously retcons some significant portions of the plot laid out in The Fall of Reach. I haven't actually played Halo: Reach in several years, but going back and reading the plot summary on Wikipedia, I can immediately see a number of differences from the events depicted in the book.
Final warning: spoilers begin here! At the beginning of Halo: Combat Evolved, the UNSC ship Pillar of Autumn is supposed to have made a random slipspace jump to retreat from a Covenant fleet—it's not mentioned in the game that the ship is retreating from the lost battle for Reach. This is how the humanity first happens upon the Halo rings (and, in fact, how the Covenant find them as well). I always had trouble suspending disbelief that a jump to some random point in the universe would result in the discovery of one of the handful of Halo rings that exist. But it turns out that the jump wasn't actually random: Cortana chose the jump coordinates based on ancient navigation symbols found on an artifact that the Master Chief had recovered from the Covenant after they invaded the human colony world of Sigma Octanus IV.
Similarly, I had trouble suspending disbelief about the concept of AI rampancy, at least as described in the game, when it was introduced in Halo 4. Frankly, I thought the whole thing was a fairly stupid plot device. But, actually, rampancy wasn't introduced in Halo 4; it was a known phenomenon in the Halo universe right from the start, and is mentioned at the same time as Cortana is introduced. I'm still not sure I'm totally on board with the concept, but it does feel less idiosyncratic now that I know that it was established long before being reintroduced in Halo 4. Originally, I viewed it as something 343 added to introduce tension to the romantic subplot between Cortana and the Master Chief, but it turns out they were actually strictly following existing lore the whole time:
Cortana was a "smart" AI, an advanced artificial construct. Actually, the terms smart and dumb as applied to AIs, were misleading; all AIs were extraordinarily intelligent. But Cortana was special.
So-called dumb AIs within the set limits of their dynamic memory-processing matrix were brilliant in their fields but were lacking in "creativity." Déjà, for example, was a "dumb" AI—incredibly useful, but limited.
Smart AIs like Cortana, however, had no limits on their dynamic memory-processor matrix. Knowledge and creativity could grow unchecked.
She would pay a price for her genius, however. Such growth eventually led to self-interference. Cortana would one day literally start thinking too much at the expense of her normal functions. It was as if a human were to think with so much of his brain that he stopped sending impulses to his heart and lungs.
Like all other smart AIs that Dr. Halsey had worked with over the years, Cortana would effectively "die" after an operational life of seven years.