Reference? Can I Get a Reference, Please?

It bothers me that novelists occasionally will quote other writers without indicating where the quotation comes from. I'm not talking about quotations that preceed text necessarily, although it's nice to have sources for those as well, rather I'm talking about quotations appearing within the text. (Stephen King is very good about identifying his liftings, I've noticed.)

I mean nothing against Jan Karon, mind you, but here is an example from A Light in the Window that I asked my mother to find for me over the weekend:

"We are not necessarily doubting," said C.S. Lewis, "that God will do the best for us; we are wondering how painful the best will turn out to be."
(240)

I really like this quote, but haven't read the book it was taken from. Is this from The Problem of Pain?

While I'm thinking about it, I thought it funny/odd/sinister that there is a character on Lost this season named Charlotte Staples Lewis.