So Threshold of Destiny is a very interesting book. I really liked the story. The Mysterium is a new, fresh idea, and a very intriguing one at that. I also liked the idea of Operation M, which I won't expand on because I don't like to give spoilers. I will say that the undercover operation and its purpose was a new spin for me. (Vague, I know, but read the book!)
That being said, I wish this book was actually split into two books, or that there had been a prequel book to take some of the pressure off this one. For me, it was a little too long, and there were a lot of details and ideas, which was great, but a little overwhelming for just one book. In some places, I felt like it was unnecessary to have so many details or some of the conversations, since they didn't really advance the characters or the plot. It made some of the passages slow, which bogged down the pace, and which made me want to skim read.
My only other sticking point was Tressa. I wanted to like her, she seems like a nice character, but sometimes she was just ditzy and a little juvenile for her age. Both of which would have been fine, but I don't feel like she grew as a character, not deep down. She faced and overcame a lot of adversity and her world was shaken up, but I feel like she was still a little too immature at the end.
I agree with another reviewer that said at times the book was 4 stars and at times it was 2 stars, I would just add that at other times it was a solid 3 stars for me. I can't wait to see how Ms. Chapel develops as a writer and what other stories she has to tell. I would recommend anyone that likes a little mystery, a little romance, and a little paranormal check this one out.