Depends. If they are good at or want to study academic English, then IELTS or TOEFL. For studying in the UK or Australia, IELTS. For America generally TOEFL, but they really should find out which marks they need and therefore which one they need before studying anyway. FCE is general English and the only one with a pass or fail cut off point, so they need to be good Upper Int level to pass it.
Ideas on books would make more sense once you've chosen one.