The two novels of Eryc Nylund are quite good. They make for an interesting read and really expand the halo universe. They are not first class literature, but that is of course not to be expected. A fan of Halo will enjoy them, and they are really quite good.
However, "the Flood" by William C. Dietz is a disaster and the absolute worst book I have ever read. Where Nylund describes the events leading to and following the first video game, Dietz basically put the video game into a novel. An ego shooter novel. Every passage about the master chief reads the same, him walking around, shooting stuff. You actually feel like in a video game, when his shield status is described; going down and recharging... When he cannot decide which weapons to take with them, because he cannot carry more than two... it's absolutely awful. The other characters are hardly interesting and almost always very stupid. Their choices hardly ever make sense.
In all, the box is a nice thing to have in your shelf, especially if you are a fan of Halo. And as I said, the first and third novels are quite good, but I would recommend against reading the second part, unless for amusement of another kind.