This is a matter of personal opinion on the net, so I will list more then
one.
For a selection of rat (and other rodent) books online, there is a BOOKSTORE run by NAPA in association with Amazon.com.
"Rats. A Complete Pet Owner's Manual", Carol A. Himsel, DVM.
Barrows 1991 ISBN 0 8120 4535-1 ~$6. Readily available at pet stores.
Nick Mays' book is much better ('The Proper Care of Fancy Rats' pub. by
tfh 1992 ISBN 0-86622-340-1) - probably because he's kept & bred rats
for years. (This book is good for an intro into genetics as well.)
There's also 'Rats' by Susan Fox Genetics: Colour Inheritance in Small
Livestock' by Roy Robinson, a who also keeps rats & other animals.
This was published in 1976 by Fur and Feather, printed by Watmough's,ISBN
0903775069.
Advanced info on all areas of genetics, aimed at scientists: 'The Genetics
of the Norway Rat' also by Roy Robinson, published by Pergamon Press in1965
- unfortunately it doesn't have an ISBN no. This book is about 750 pages
& is not for the faint-hearted. It has a bit of info on colour breeding
etc., but is on all areas of rat genetics.