Your mom’s HR representative is going to be able to explain to her how the tuition benefit works. None of us has that information. What will your textbooks cost? We don’t know- are you majoring in English Lit (so you’ll be reading lots and lots of novels, most of which you can buy second and third hand and get out of the library) or majoring in Chemistry in which case the books are expensive and you’ll want to own them, not get out of the library because you’ll need them all semester.
So get a handle on your tuition benefit and then take a look at the Wash U page on the website where they outline the basic cost structure.