Statement A is definitely out of context, because we don't know what was the ranking of Thursday (3rd highest day, 4th or ???). So, IMO, it doesn't help.
Statement B is tricky.
We know that Saturday was the busiest day and the store sold 38 books and we know that Friday is the second busiest day. Also, we know that each 7 day of the week the store sold diff. number of copies.

Here is two tests:
1. First test, we decide that on Friday they did not exceed 11 copies. We choose 11.
Saturday: 38 Friday: 11 Thursday: 10, W 9, Tue 8, Mon 7 Sun 6. Total = 89. So, given that Friday they sold 11 copies, the combination of the other 5 days will give maximum of 89
2. Second test, we decide that on Friday they did exceed 11 copied. We choose 12.
Saturday: 38 Friday: 12 Thursday: 10, W 9, Tue 8, Mon 7 Sun 6. Total = 90.
So looks like exceeding 11 copies on friday (12 in our example), satisfies the problem.

I think my method is pretty long, I believe there should be easier way to solve it.

Therefore, asnwer is B

actually this is a very easy question

Statement I : clearly insufficient , Y ? We are not given the max number of book or the minimum so the maximum can take a lot of values hence its insufficient

Statement II : we are given the maximum and we are also told that on each day the shop sold a different number of books so the maximum number of book which might have been sold on Friday = 37

But we know that each day the shop sold a different number so

90-38 = 52 if we divide this by 6 we get approximately 9 but the shop didn't sell equal number of book on the six days so definitely the number of books sold on Friday must be greater than 11

Hence statement II is sufficient

Answer is B

the OA is B..

but which statement in the Question tells that each day they sold different books.
"it sold different numbers of copies on any two of the days"