Between 2.5 to 3 stars. I bought this one on sale as a Kindle Daily Deal, sort of the equivalent of picking a book off the shelf at a library and reading it just because it was available. The first half of the book was better, because at that point there was still so much potential to where the story would go. I thought the resolution was only moderately interesting. The writing style was not to my taste; I don't often like reading in the present-tense. Also, the author had this annoying habit of writing the protagonist's dialogue without quotation marks and without indicating that his words were spoken out loud. To its credit, I did want to know how the book ended and it was not what I predicted. I wasn't really attached to any of the characters though. Overall I probably wouldn't recommend this book, although as a light and simple read (to cleanse the palate between intense and thought-provoking literature) it did the trick. 
 PS It irked me a bit that the name Penumbra is actually a medical word describing the region in an ischemic stroke in which the damage is reversible and function can be recovered if blood flow is returned. I suppose the word has a nice bookish ring to it but it was sort of weird for me to read it as a name!