Calling preflop raises

Hi,
I'm studying the small stakes hold 'em book, and I don't quite understand some things about pre-flop play.

How do I decide whether to call a raise that's come behind me after I've already called the BB? He makes the distinction between cold calling a raise, and calling a raise after I'm already in the pot, but I'm not clear on how to decide when to do that.

Also, there are some pre flop hands that he recommends cold calling a raise in early/middle position, but the same hands in late position can only call if there are 3/4 in the pot (if at all). I don't understand this. Seems to me if you're willing to call in early position, you'd be more willing to call in late position. Can someone help clear this up for me.

Thanks for your help
Rob