Even if you've not read any of the Nick Stone series (which you should do), you could pick this book up and read Crisis Four as a standalone.
By his own admission, McNab's protagonist isn't hired for his cerebral capabilities, nor his charitable heart, but for his resourcefulness and ability to get a job done. However this time Nick Stone's mission is to kill one of the few people he's ever actually cared about.
McNab follows his usual pattern of beginning the story some years earlier to build a skeletal background to the main plot, and little by little he reveals details that show real technical novel-writing capabilities, building to an explosive and breathtaking (pardon the cliches) conclusion. His character development (of all the characters) is good, and, even if you never actually get round to liking him, you'll at least get some more insight into Nick Stone's inner workings.
The storyline is up-to-date, and relevant - this becomes more apparent now that we're a decade on from when it's set.
McNab's colloquial and somewhat chatty style won't appeal to everyone, but he writes "in character", and you really get the impression that that's what Stone is really like.
Andy McNab is a writer with considerable expertise in his subject matter. After all, he lived the Special Forces and Deniable Operative life. This shines through in his description of trade and field craft. He manages to balance giving enough detail to enable the reader to imagine for him/herself, without drawing attention away from the plot, or boring the reader.
I would definitely recommend this book, and would encourage you to read the Nick Stone series in it's correct order, to fully comprehend the skill with which McNab develops his character through the years.