Mishca’s heart is not her own, and then she falls in love. [You have my attention - good opening line.]
After graduating high school and undergoing a life saving heart transplant, Mishca Richardson [If this is YA you need to give us the character's age early on[ hopes that she can finally have a semblance of a normal life. But her new heart appears to have consequences as she is plagued by post operation nightmares and a sudden yearning to find her birthparents, which has never been an issue for her before. [Interesting premise - I want to keep reading]
Mishca’s search for birth parents leads her to Ryder, a fellow adoptee who is unfazed by her health issue and lack of experience with boys. While her romance with Ryder blossoms, other things in her life are not going according to plan as she discovers no-one has ever been able to trace their birthparents with the agency she was adopted through. Then Mishca’s whole world is turned upside down when she goes to university and meets her literature professor Colin Read.
But the gravity of her situation is worse than Mishca realises. She is secretly being watched by a man called Wirth, who is responsible for her dream, knows the truth about her past and has plans for her future.
[A nice amount of detail, and well broken up - I didn't feel like I was being given too much]
Mishca is a YA Speculative Fiction [why is it spec fic? It doesn't sound like it necessarily fits into that genre], complete at 50,500 words. It is designed to be book one of a series. While Mishca and the series are set in Australia, I have undertaken market research on a YA writers’ community, Inkpop, by posting a portion of an early draft the story. I received nearly 700 comments, predominately from American teens, and the feedback shows that Mishca would be commercially successful internationally if published. In June Mishca was actually voted in the top five favourite projects on the site out of more than 25,000 pieces of work by the members. [Good to include this - it's useful information for agent/publisher as it shows you've connected with a potential readership, and connection is a good thing!]
I am looking for an agent who will work with me, through editing and advice, to establish myself as a long-term career author. As per your submission guidelines, I have included the first ten pages of Mishca below.
Thank you for your time and I look forward to hearing from you soon.
Overall: a very good letter. You have an interesting premise for the story and you've let it speak for itself. The only real quibble is with the genre - it sounds like it should be plain old 'young adult fiction' rather than 'YA speculative fiction'.
What you haven't mentioned: the author included some indication of where she would personalise the letter for the agent/publisher it's addressed to, and I removed the references so they didn't interrupt the flow of the letter.