A margin is the space between your text or illustrations and the edges of the paper (or virtual paper in an ebook). I mentioned my rule of thumb: a margin at the side of a page should be big enough to fit an adult human thumb without covering any text or illustration.
Each page has four margins, and they can be the same or different. It’s common for vertical margins to be larger than horizontal margins, and sometimes the top and bottom margins are not the same size. This is where the book's formatter gets to make an aesthetic judgment. Small margins make a book look lousy and hard to read. New designers and cheapskates often maximize the number of words on a page, so fewer pages will be needed and a book can be printed for less money. (A printed page costs about a penny, e-pages cost nothing.)
White space demonstrates extravagance and implies wealth. When I was a child I was advised to eat everything on my plate. When I was a teenager I dated a wealthy girl who had been taught to always leave some uneaten food on her plate so no one would think she actually needed the meal. White space is part of the paper you choose not to print on. If your primary consideration is to get the most for your money, you would leave as little white space as possible.
Because of its uniform line length, justified text lacks some of the negative space that flush-left text provides. Experiment with other ways to add negative space to a page. Larger margins can help. Extra space between paragraphs adds negative space which makes a page more attractive, but also makes each paragraph look more independent rather than part of a unified “whole.”
Your publisher or printer can tell you the minimum margins for the page size you’re planning to use. A common minimum size is ½ inch on all sides. You can choose to have bigger margins than the minimum, but not smaller.
• If you have either large pages in a slim book or a spiral binding it’s good to have smaller margins on the inside of a page (the gutter) than on the outer edge. This can make the three vertical white strips (left, center and right) look approximately the same.
• In thick books the inside gutter margins often dissipate as they curve into the binding With the common 6-by-9 page and a book of about 300 pages I like to use the same-width margins on left and right.
• When a printed book has more than about 500 pages, it’s a good idea to provide additional gutter width to compensate for the white space that dissipates into the binding. Your printer or publisher can advise you.
• If your book is going to be e-only, you don’t have to think about gutters.
A printed book with large pages simply has more room for white space than does a book with smaller pages. In newspapers where space is fought over by editorial and advertising departments, text gets less white space than in books.
[below] Some good advice from 1907.
[below] Without sufficient negative space, a page seems overstuffed and it repels -- rather than attracts—readers.
[below] When leading is too large, the negative space dominates the text.
[below] Here’s a much nicer version, with full justification and hyphenation.
[below] When a graphic element is inserted within text, make sure to provide adequate white space around it. Compare the upper and lower photos in the page shown. The amount of white should be proportionate to the size of the graphic, but there is no specific rule. The more space you provide around a photograph, the more important it will seem to be. The default spacing in Microsoft Word is .13 inch. You probably should not go below .1, but if a photo includes its own white or light border you can get closer without crowding.