"I am am writing to you because you represent X, whose {book name}I admire."
"this book is like book X, only with a more Y vibe"
Sentence one -- you've done your homework, you know agent reps book X. This cannot hurt, but i suspect could be worded better
Sentence 2 - you have thought about where your book fits into the market. Bonus!
"I am writing to you because I am author X's biggest fan and you are her agent and I wuv you!" Yeah, no.
Tie any connection back to your book, be professional. This is not about how you love X's book. It's about yours, and that you have researched etc.
Dear Mr Agent
I am writing to you because you represent X and her fantasy noir novels. {pitch} I believe my book(s) would interest readers of X, Y and Z because Pi*.
Only, I dunnow, betterer.