My niece is starting her freshman year of college. She packed her books to take and texted me that she had her Bible, apologetics books, and Jane Austen books, and asked if that was enough. I said that's pretty much the essentials.
Here are the apologetics books she packed because she thought they were good preparation for college:
Tactics (of course, she'd be disowned otherwise)
C.S. Lewis collection
And she has Doug Powell's iWitness app collection on her iPad.
Of course, there are lots of other great books to prepare Christian college students for challenges, but these are a great start.