A fraction is a part of a whole.
The whole is in 6 sections. 5 of the sections are shaded. That means the shaded part is 5 out of the 6 total, so the fraction should be 5/6.
Therfore, Nathan read 5/6 of the book during the second week, finishing up the book because he read 1/6 of it the first week. He read 6/6 of the book, the whole thing.
Hope this helps! :)