He read 400 pages from the 500 page book because 500 x .8 = <<500*.8=400>>400
He read 200 pages from the 1000 page book because 1000 x (1/5) = <<1000*(1/5)=200>>200
He has read 600 pages in total.
He needs to read 200 more pages to reach 800 pages because 800 - 600 = <<800-600=200>>200
#### 200