Solution: Let the purchase price of the first book be X yuan, the first book be 100/x, and the second book be x+0.5 yuan.

(x+0.5)*( 100/x+ 10)= 150，

X 1=2.5，x2=2，

Because 2.5+0.5 = 3 >; 2.8, the purchase price of 3 yuan, the boss 2.8 yuan can not sell, does not meet the meaning of the question, I give up.

So x=2.

So I bought 50 copies for the first time and 60 copies for the second time.

When four fifths of these books are sold, the sales amount is 60 * (4/5) * 2.8 = 134.4 yuan, and the remaining books are sold: 60 * (1/5) *1.4 =16.8.

So the total sales is134.4+16.8 =151.2 yuan.

So the boss earned151.2-150 =1.2 yuan.