Should you use double space or single space when publishing a book?

Go to your bookshelf, or a library. Select a book. Open it.

You will see:

Books, except in a teeny tiny handful of niche cases, are always single-spaced.

Books always have one space after a period, not two.

Books are set with the type justified.

Books use smart quotes, not dumb quotes.

Books use em dashes to indicate breaks, not two hyphens in a row. Always “—”, never “--”.

Book design is an extremely specialized and surprisingly difficult form of graphic design. If you don’t lay it out correctly, your readers will know. They might not know how they know, but they will know. A poorly typeset book screams “amateur.”