Re: My book doesn't answer this question
I think the usual rule is that it's fine to mix them as long as neither of them has been frozen. Maybe that's what you were thinking of with the different temps thing Colleen?
ETA: I might be wrong, but FWIW I always pumped right into the refrigerated bottles for my second and third pumping sessions and never had any problems.
“We are not put on earth for ourselves, but are placed here for each other. If you are there always for others, then in time of need, someone will be there for you.”