If your book is done in columns or with a small page size, then it is usually better to hyphenate words. Your right hand margin will be more even and your page layout will look better. Most word processors will automatically hyphenate words for you, but you need to proofread your book carefully to be sure that the hyphen didn’t end up in an unfortunate place. There are a few words that when split in the wrong place make two separate words, like ‘the-rapist’ and ‘thin-king’. While these “howlers” could be funny to some people, they could be upsetting to others. It is better to be safe than sorry, so look for the hyphenation howlers before you approve that final proof copy.