The margin size for a book generally depends on the type and size of the book, as well as the format. Generally speaking, the standard margin size for a book is at least .75 inches all the way around. For example, if the page size of your book is 8.5” x 11”, then the margin sizes would be .75” at the top, bottom, and outside of the page, and .5” for the inside.
If you are making a hardcover book, you may choose to have larger margins for aesthetics and to allow for a thicker spine. If that is the case, the standard margin size for a book can be 1.5" or even 2" on all sides.
However, you should keep in mind that the margin size of your book should never be so small that the text or images run off the page. The text should remain within the margins and look professional, so adjust your margin size accordingly.