I struggled with this one. On the one hand, it's actually quite good, for the most part; it's the book (or part of the book) where the Rifters series goes into full-on postapocalyptic mode, and Watts does that quite well. Much of the book is a grind, but it's a nastily evocative grind. Given that Watts had to split βehemoth into two parts it's a quick read (and he's right, this one and β-Max are different enough that the split isn't super jarring) but there's one unexplored question that kind of nagged at me a bit by the end. Which isn't more abrupt than plenty of books like this, really.
And then... SPOILERS, obviously, but this is the book of this series I could least recommend with a clear conscience. The villain is ultimately who the reader probably suspects it was going to be, from β-Max if not before, but I don't think we needed to have him capture, rape, and torture (presumably to death, although that's left unclear, for fuck's sake) another character. Watts' writing in those sequence doesn't feel prurient to me, but I was wincing throughout, and again, this stuff doesn't actually hit any personal triggers for me. I guess I appreciate him not soft pedalling how and why this guy is so horrible (and the twist, the reason said villain doesn't end the world at the end, is queasily compelling), but those sequences still just feel needless. The character he tortures could have figured out what was going on in other circumstances, or that information could have been presented to the reader in some other way. Watts is partly making a point about the mechanistic nature of human personality (one I don't fully agree with, which means he no doubt would think I'm softheaded, which is fine) but I really feel like the good parts of that aspect of the book could be wholly preserved without the rape and mutilation in this book. I'm strongly tempted to rate this one lower; those scenes almost made me put the whole thing down. I'm glad I didn't, ultimately, but note that this book is not going to be pleasant or even healthy for some people to read. If Watts had found some other angle to take with that character, not only would I be more able to recommend the series (and this book would probably be a solid four stars for me), but it would be way more compelling. You've got a guy who's willing to damn the whole world just because he's conscience has been disconnected and it lets him keep doing whatever he wants... and what he wants is literally the most predictable, cliche thing in the book. Completely separate from any distaste I felt reading those passages, the character ultimately feels like a failure of imagination and a huge missed opportunity.
For example, look at the thought process Ken Lubin ultimately uses to defeat him; now there's a case of Watts taking his ideas and playing with them to both interesting and disquieting effect. Lubin is a great example of what's good about the dark parts of the Rifters series, mainly because his urges and psychology and actions feel more convincingly worked out. I am glad I read the series, and I think Watts is a hell of a writer; I'll be thinking about Lubin and Lenie Clarke for a long time. As far as postapocalypses go, this is a very well done one, and I think the ending is really well done. I could only recommend this one with some pretty heavy caveats, though.