I'm a student trying to input cost of books. What should I input if I don't know the exact amount?

If audited, the IRS does not award any deduction you can't prove with adequate documentation.  So, while you could try and make a close guess, if you were audited, it would probably be disallowed.  The risk is on you.

Generally, the cost of books is not included in box 1 of the 1098-T. But it could be. If not included, then when you purchased those books I assume you did not pay for them with cash. You used a credit or debit card. So the transaction should be on a bank statement. If you wrote a check for for the cost, then it should be on a checking account bank statement.

If you can't prove the expense, then you claim that expense at your own risk. If audited and you can't prove it, the IRS will disallow the expense and that could result in you having to pay some of any refund received, back to the IRS with interest and penalties.