“I am writing to you because you represented The Book of the Dead by Douglas Preston and Lincoln Child, and I feel my novel, with its mixture of crime and supernatural elements, is similar to the Pendergast novels,” is one sentence that needs to be two. What this efffectivley is a run-on sentence and no good for a reader. It can be split in two and rearranged for more effectiveness. How about this:
WHAT ROUGH BEAST is a 174,000-word horrow novel. With its mixture of crime and supernatural elements, this story is similar to the Pendergast novels your agency represents.
The word count has significantly been reduced. This is just a suggestion for the first line.