Book/styles.css

.book-page-container {
	counter-reset: pages 0;
}

.book-page-container > div {
	position: relative;
	margin-bottom: 2rem;
}

.book-page-container > div::after {
	content: '(Page ' counter(pages) ' of ' attr(data-maximum-pages) ')';
	position: absolute;
	bottom: -2rem;
	text-align: center;
	width: 100%;
	border-top: solid 1px gray;
	color: gray;
	counter-increment: pages;
}