I really liked By the Sword, which was my first book in the series. I don't recommend you start there, but you certainly can. Everything was explained well enough that I didn't feel lost. I felt like I was missing something, but I didn't feel lost.
Ground Zero is the next book in the series, and it continues the strong story. Wilson has made it plain that the books in the series will no longer be self-contained, as he is leading up to the finale (there are two books left after Ground Zero, I believe). That's certainly the case here. I wouldn't say it ends in a cliffhanger, but there are so many story threads leading into the next book that it might as well have.
It takes a talented author to incorporate the events of 9/11 into a dark fantasy series and not make it look in bad taste or tacked on for shock effect. Wilson is that author. My favourite part of the book? It uses the 9/11 Truther conspiracy theorists, incorporates them into the narrative, and still makes them look like the idiots they are.
It basically does that by saying (within the world that Wilson has created, not for real) "the Truthers are onto something, but they're way off on what it is."
I love it!
My review has now posted on Curled Up With a Good Book.
"A decade after the horrific events of September 11, 2001, the world is going crazy. The dark plot that Jack has been fighting seems close to fruition, and the pieces are falling into place. In the meantime, Jack's childhood friend Weezy Connell has joined the 9/11 Truther movement, piecing together little facts and discrepancies that seem to lead to an inevitable conclusion: the true story of 9/11 has not been told yet. She posts her conclusions anonymously on the Internet, but somebody knows who she is. In desperation, she calls Jack, not knowing that he is the same Jack she knew all those years ago. Jack finds himself immersed in the Truther movement and discovers that they have no idea how far off they are while still being right. What's really behind 9/11 - and many other recent events - is part of a war millennia old, a war that may be catching fire again soon."Wilson's imagination is endless, and Jack (known as "Repairman Jack" because people hire him to come and fix things) is a wonderful character. He's doing the best he can in a conflict between Good and Evil, caught in the middle and just trying to tread water and keep his loved ones alive.
The book is only slowed a bit by Wilson over-explaining the whole Adversary Cycle. That may be helpful to new readers, but I like to think that even they might say "can we please get on with it?"
I would definitely not start the series with this book. Though, again, you would be able to understand what's going on. It just wouldn't mean as much to you. At least start with By the Sword, if you don't want to start at the beginning. That's where I started, and it certainly hasn't hurt me.
Read the review for more, and let me know what you think! And if you're a Wilson fan, I'd love your thoughts as well (though don't spoil the next book for me...I know it's out, but I haven't read it yet).