Ezio - may I recommend the book, the scuttling gourmet by Alison Campbell.
Very very highly regarded in the fancy and it's all about rat nutrition and how to make your own mixes - if you wanted to.
Ratrations essentially has the mixes ready to go but some people prefer to make their own.
Also the book common diseases of the fancy rat by the NFRS. It's written by Ann Storey who is arguably the UK's most informed rat lady and also a biologist type person! Knows all there is to know about viruses and bacteria!
she was a life saver to me in the past when I kept rats. She pretty much heads up the fancy in the UK xx