In order that two  books on Hindi are never together , we must place all these  books as under :

        ` X E X E X E X .........................X E X`

where E denotes the position of an English book and X that of a Hindi book.

Since there are 21 books on English , the number of places marked X are  therefore , 22

Now , 19 places out of 22 can be chosen  in  `22_ (C_19)`

= `22_ (C _ 3) = (22 xx 21 xx 20)/(3 xx 2 xx 1)` =   1540 ways.

Hence . the required number of ways  = 1540.