Shadowland (2002) was Wilson's last work of fiction, and nearly his last significant work. (The exceptions are his autobiography, the excellent but somewhat embarrassingly titled Super Consciousness, and a collaboration with his son.) He is still very much at the top of his form here. (A 2011 stroke laid him low, and he died in 2013 following back surgery.)
Readers are somewhat divided about it, however. The problem is that the series begins as science fiction with a fairly conventional premise, but here has a significantly different feel, because of his introduction of such entities as nature "elementals." Wilson himself, I think, would not have seen a sharp difference in the direction he takes in Shadowland, but many readers will. The reason he feels comfortable with this direction is because of the years of research he'd done at that point on the "supernatural," and the point of view he had developed with respect to "supernatural" phenomena.
So, what is the truth? Does it provide a satisfactory conclusion to the series? (There are three novels that precede it.) The first point to note (which none of the reviews do, in fact, note) is that it wasn't really intended to conclude the series. In the Acknowledgments, he comments that his publisher had encouraged him to work on a novel what would probably actually have concluded the series, to have been titled "New Earth." It's extremely unfortunate that he didn't live to write it, because it would have been Wilson's fictional statement of what he conceived ultimate human destiny to be.
What we do have in Shadowland, however, is a quite satisfying conclusion to the preceding three novels, provided that the reader is able to make the transition from the more conventional premises and characters to the new sorts of entities that he/she will encounter here. In many ways, they are actually quite a bit more intriguing than the spiders that provide the focus in the preceding novels. That's not to say that I consider the existence of entities of this kind to be at all likely (I don't), but they are here convincingly constructed as fiction, particularly in the second half of the novel.