Dragons'Nests and Fire Birds is a 76,000 word fantasy aimed at 12+ readers. It is intended to be the first in a series. [It's more important to tell me what the story is here than tell me it's the first in a series - the series won't happen at all if you can't grab someone's attention with the story first.]
When Nell and her sisters inherit a box of jewels and an old Emporium, Nell hopes to make her dream come true and open a spice shop. However the Emporium has its own scruffy (and condescending)'guardian'- Brendan, and Nell keeps seeing dragons. Overhearing her Mum and Brendan arguing about a vengeful Merchant, Nell covertly follows Brendan. She finds herself in the magical Lands of Lost Lore. Nell realizes her childhood fantasies of dragons flying and mermaids singing are truly her earliest memories.
[This would be a better opening paragraph.]
Returning to Earth, Nell confronts her Mum.
Nell learns that her dad, a werewolf from the Lands, was killed in a pack dispute. Nell's step-dad was killed saving dragon nestlings from the evil Merchant.
Nell and her sisters decide to explore the Lands with disastrous consequences. When they are all captured by the Merchant only Nell manages to escape.
In return for Nell's sisters the Merchant demands a mermaid's tear, a griffin's claw and a phoenixes' feather. Nell and Brendan embark on the arduous quest.
[Condense this second paragraph - we don't need that much detail about the story in the letter, as there is usually a synopsis to read if we want detail.]
I have had two young adult fantasy novels published, Kingdoms of the Seventh Pool (Holy Angels, 1998) and Lumi's War (Holy Angels, 2001) [This information can come earlier. In your opening paragraph, you could instead say, 'I am the author of two published young adult fantasy novels. Dragons' Nests and Fire Birds is my third. It is a 76 000 word story aimed readers aged 12 and over.' This would get an agent/publisher's attention more quickly. Is there anything more you can tell us about you - how long you've been writing? Why you love YA fantasy?]
General feedback: You could sound a bit more confident. Blow your trumpet early - about your previous books, and about this story. There's no sense here that you really want to encourage me to read this manuscript. What's so good about this story? Why should I want to read it over all others? Describing the storyline isn't enough. Tell me why it's great. If you don't believe it's great, no one else is likely to ...