Well, I've had my problems with this book. Several problems even. But I will get to that in a minute. Lets focus on the good stuff first.
The general idea of Sentinel is pretty cool, I think. It's a nice story to read and most of the characters are pretty cool. The writing's nice. But still.
The world Nicholas, the main character lives in is kinda like in the Shadowhunter world. There's normal people and then there's not so normal people, but the normal people don't know about them. These not so normal people are the Sentinel. Nicholas' parents were Sentinels and after they get killed, the boy gets sucked into this 'other' part of his world. This brings me to the point which annoyed me the most in this book. Nobody cares to sit Nicholas down for a few hours and explain this damn cool, exciting new world to him. Of course this sucks for Nicholas, who basically has no idea what's happening to him, but it also sucks for the reader. You have no idea what's going on. A lot of things got clearer towards the end, but I still can't say I really understand what's going on in this world. I don't know what exactly Sentinel are, where they came from, what they're doing - apparently they're only there to somehow protect the world from these evil creatures - I don't know much about those either.
I'm sure the world Joshua Winning has created here is wonderful and yes, it sounds promising and exciting, but a little bit of worldbuilding would have made this story one billion times better. I hope this gets more attention in the sequel.
Other than that, I found that Nicholas was just super annoying most times. Like, seriously boy, you have no idea about this new world, a woman tells you that you have to stay inside the house at all costs - and you decide to go outside?! More than that, doing so, you almost get yourself killed at some town - and as soon as you get back to safety you decide you want to go back there?! Yeah, sure, you're lucky that didn't cause you any problems in the end... /heavy sarcasm off
Now, this may sound as though I didn't enjoy the book at all, but I did. I really did, it was a nice little read (which took me ages to get through but that was because of school), it just wasn't especially capturing or exciting or anything...
Sentinel is a book with a great concept which is still a little poorly executed. There was so much potential the author simply didn't work with and I really hope the sequel, which I will most definitely be picking up makes up for that a little bit.
I'd recommend you just try the book and see for yourself, it's short, it shouldn't take you that long to read and it's not a waste of time.