So we have 1 - 1/8. That is, one whole book, and 1/8 has already been read. What you need to do is convert that whole number into a fraction in order to have the same denominator:
1 * 8/8 = 8/8
Now that it's converted, we can subtract:
8/8 - 1/8 = 7/8
So Ralph has 7/8 of the book left to read.