From the title, we can know that there are 56 books in grade six, 28 books in grade four, and 42 books in grade five. So the sum of grades 456 is: 28+42+56 =126; And 126 is only 60% of the newly purchased books, so the newly purchased books are: 126 divided by 60%=2 10. So the number of new books is 2 10.

Suppose: the newly bought book is W, grade four is A, and grade five is B; Then it is:1) 2: 3: 4 = a: b: 56,2) a+b+56 = 60% w, 3)(A+56)X 1/2=B, and it is calculated that A=28 and B=42.

That is, the school has bought 2 10 books.