20% whitespace in text body, margins should be around 40% of the width, for English textbooks, preferably including a column down the left side of the page, or split over both sides, or down the right side as a last resort if sufficient room on the left is not available).
A suggested layout for English textbooks involves 20% empty space in the text body and margins that occupy around 40% of the width. This design often includes a column along the left side of the page, or divides the content between both sides, with the right side used as an alternative if there's not enough room on the left.