It's a myth that you can't mix raw and kibble. Digestion rates are different but it's not like the two got numbers, as in the old fashioned butcher shop, and raw got numer 2 and has to wait for number 1 to pass through. All foods digest at different rates and whatever is digested first passes through first.
If you have a dog that like to nibble at it's food throughout the day, instead of gobbling it all up in seconds (Lab here
) then there may be some concern as expressed above. Especially in hot weather.
Which book do you have? I like Dr. Pitcairns second edition. In the first he uses grapes.