Kate was in the UK on her first big book tour when Newsround caught up with her.
Who are the main characters in the book?
Well there's Despereaux himself, a most unconventional mouse who's not aware of how unconventional he is. He exists on a different level altogether.
He loves music and books and he falls in love with a human princess called Princess Pea.
The Princess is a pretty standard issue princess except her mother died.
Roscuro is a rat living in the dungeon who becomes cursed with the love of light and journeys up into the castle and becomes smitten with the whole concept. He then ends up inadvertently killing the Queen by landing in her soup.
And the fourth character is Miggery Sow, a girl who's had a life of nothing but misfortune. Her father sold her and she's had a hard life and yet she has this ridiculous, wonderful dream to become a princess.
How do they all come together?
With the rat and the mouse it's oddly that love of light that they both share that brings them together.
Every action has a consequence... because the rat falls into the soup and kills the Queen, the King outlaws soup and because soup is outlawed Mig ends up at the castle.
And because the rat had a bad look from the princess he vows revenge on her.
That's how they all ultimately end up together, although I don't think all four are together until the end of the book.
Do you believe that everything you do affects something else?
The smallest act of kindness or the smallest act of hatred can have these huge consequences and we can't foresee them.
You never know who's heart you're breaking - sometimes you're breaking someone's heart without even knowing it.
Is Despereaux the mouse based on anyone you know?
No. I wish that I was more like him - I'm not as courageous as he is.
He's so different and yet it doesn't really bother him that he's different. Right until he's right up in front of the Mouse Council he doesn't really realise that he's in trouble.
I admire him but other than loving light and books I'm not much like him. I don't think I would be as brave as he was.
Lots of the characters have been separated from their parents, do you think this is an important issue?
It's a basic fear - if it hasn't happened it's still a fear that most kids have that it will happen.
The first two books that I wrote they have missing mothers in and when I sat down to write this one I thought 'Whatever you do don't let the mothers be gone!'
So much for that! You've got dead mothers and the one that's alive isn't much to write home about - Despereaux's mother Antoinette is a pretty lousy mother.
I grew up in a single parent home and I think that's part of the reason that's there.
Also, in any book when you get rid of the parents the action kicks in. Then all the fun can start.
Should fairytales always end nicely?
Fairytales end in a mentally satisfying way. I don't know that it's necessarily happy ever after.
I remember when I was a kid I think I got really frustrated with the books I read because things were tied up so neatly and didn't really reflect the truth of existence as I perceived it as a kid.
So I think they can be happy, but it's a different happiness than we anticipate. Everyone ends up OK but nobody gets exactly what they wanted.
Will there be more tales about Despereaux to look forward to?
I don't at this point think that I will. Who knows? Maybe. It was awfully fun. I wouldn't say no.
Can mice really talk?
No I don't really think that mice can talk but I think there are more mysteries in the world than there are answers so anything is possible.
Are rats that bad?
I'm sorry to contribute to the continued bad representation of rats!
A lot of kids now have rats as pets and they say that they're really intelligent and they've great personalities.
But I've still got that 'nasty, dirty' attitude towards them!
And your fave animal?
Dogs - I'm a big dog fan.
The Tale of Despereaux, published by Walker Books, is available in bookshops now.