I have to hit the books.  I haven’t cracked a book all week.

These figures of speech refer to studying.  ‘I have to hit the books’ means ‘I have to study’.  To ‘crack a book’ means to open it.  ‘I haven’t cracked a book all week’ means the student has not opened a textbook to study anything during that week.  The two phrases are similar and can be used together, or separately.