I agree there - it's almost always worth looking at the very start of a Pratchett book to get a key to understanding. Terry will often start a book with a few apparently throw-away lines that you read and forget, but they are almost always relevant.
They chased the dogs and bit the cats, they ---
This is the first line of the book and an almost direct quote from Robert Browning's The Pied Piper of Hamelin.
If you haven't read it before then it's well worth it as the story of the Pied Piper is relevant to this book.
As Sharlene points, Maurice says it's just a book about rats and people. That's true - there are people acting despicably - the rat catchers in particular - stealing from there fellow towns people - letting them starve so that they can make a lot of money. Yes they might be under the influence of the King Rat, but maybe the King Rat is just getting them to do more of what they were already doing. The King Rat was created by these two so it's possible they were up to tricks long before that.
And the rats are trying to be people. They realise that with intelligence come responsibility and they are rejecting the rat ways as being morally and hygienically wrong. Hamnpork, the vicious old leader is losing his power over the rats as they look to intelligence rather than physical strength. Female rats are refusing to mate with him - I imagine that is almost unheard of in a rat society. Normally females would be only too pleased to mate with the dominant male.
But the females value intelligence over brute strength and want that for their offspring.
So yes - it is a book about rats and people - and a cat that tags along with them and (although he won't admit it) comes to care deeply about them.
Then Malicia says it's a story about stories - and from her point of view it is. This is something that Terry explored in Witches Abroad - the way that stories have a habit of repeating themselves. Malicia herself is straight out of an Enid Blyton 'Famous Five' book and she certainly sees life that way.
I see her as a lonely girl - partly he own fault, but a lonely girl who has found an escape in books. Books are exciting and as far as she is concerned life isn't. I can empathise with her somewhat there as I recognise myself as a kid there as well.
Edit to change name.
"Goodness is about what you do. Not what you pray to."