I used Case Files, but as a resident I use mostly Up to Date or Pocket EM.
For what it's worth, someone above had a good point. Emergency Medicine is a whole different set of thinking. It's basically, work up a differential while trying to rule out all of the bad things, or the things that can kill you.
For example:
22 year old man comes in to ED looking anxious and complaining of chest pain. You think he's just having a panic attack, but your job is to (by history, physical and/or labs, imaging) convince your attending why you don't think he has:
a) acute MI
b) PE
c) pneumonia
d) pneumothorax
e) Booerhave's (sorry for the spelling)
or
50 year old woman comes in with chronic low back pain. Your job (if you choose to accept it), is to work up the patient and convince your attending as to why you don't think she has:
a) cauda equina
b) epidural abscess
c) abdominal aortic aneurysm
etc etc etc, and so on... Unfortunately (or fortunately, depending on how you look at it), a lot of Emergency Medicine is saying to patients, "We don't know what is causing your pain, but we do know that it's not an emergency. Peace out." One of the biggest weapons in your arsenal will be having a broad differential diagnosis for common chief complaints (again, as mentioned above).
Hope that helps frame your thinking!