Any really good disease book will help...clinicals will be where you learn a lot...when you start rounding with doctors it's a wake up call....get on the good side of any one you can and many of them will be willing to teach or explain why they are ordering what they are ordering....
Personally, I got a 10 minute lecture about the immune system during my first clinical round...it was me, staff nurse and charge nurse....staff nurse had to go do an surgical admit....charge nurse had to go answer page....was me and doc all by myself! (with round, suprised eyes)..."What's his WBC level?" "Um...let me see doctor, it's right here on the lab sheet, slightly elevated at xxxx" "Ok, well let's start him on x, x, x....." Doc, I'm just a student I can't take your orders but I'll make sure the staff nurse charts them....ok.
At station "Do you know why his WBCs are high?" "No sir, why is that?" "Well during an infection WBCs go up and blah blah blah(spaced out here for about 8 mins, nodding and going "hmmm." "that's why we're doing x...." Ok, thank you very much!

Seriously, ask as many questions as you can and don't overlook the CNAs..they know a lot of the background info!!!