WordPress Menu Order Sort for Adjacent Previous - Next Single Post Navigation - https://amethystwebsitedesign.com/wordpress-menu-order-sort-for-adjacent-previous-next-single-post-navigation

functions.php

<?php
// Do NOT add the line above when you copy.

/** Add previous/next post navigation on book posts.
 * http://snipplr.com/view/74493/adjacent-post-by-alphabetical-order-in-wordpress/
 * /wp-includes/link-template.php 
 * line 1608
 * $where = apply_filters( "get_{$adjacent}_post_where", $wpdb->prepare( "WHERE p.post_date $op %s AND p.post_type = %s $where", $current_post_date, $post->post_type ), $in_same_term, $excluded_terms );
 * line 1620
 * $sort  = apply_filters( "get_{$adjacent}_post_sort", "ORDER BY p.post_date $order LIMIT 1" );
 */
add_action( 'genesis_entry_footer', 'amethyst_book_post_nav', 16 );
function amethyst_book_post_nav() {

	if ( is_singular( 'book-post' ) ) {
		global $post, $wpdb;
		
		// Section 1.
		function filter_next_post_sort( $sort ) {
			$sort = 'ORDER BY p.menu_order ASC LIMIT 1';
				return $sort;
		}

		function filter_next_post_where( $where ) {
			global $post, $wpdb;

			$where = $wpdb->prepare( "WHERE p.menu_order > '%s' AND p.post_type = 'book-post' AND p.post_status = 'publish'",$post->menu_order );
			return $where;
		}

		function filter_previous_post_sort( $sort ) {
			$sort = 'ORDER BY p.menu_order DESC LIMIT 1';
				return $sort;
		}

		function filter_previous_post_where($where) {
			global $post, $wpdb;

			$where = $wpdb->prepare( "WHERE p.menu_order < '%s' AND p.post_type = 'book-post' AND p.post_status = 'publish'",$post->menu_order );
			return $where;
		}

		add_filter( 'get_next_post_sort',   'filter_next_post_sort' );
		add_filter( 'get_next_post_where',  'filter_next_post_where' );

		add_filter( 'get_previous_post_sort',  'filter_previous_post_sort' );
		add_filter( 'get_previous_post_where', 'filter_previous_post_where' );

		// Section 2.
		$previous_post = get_previous_post();
		$next_post = get_next_post();

		echo '<div class="adjacent-entry-pagination pagination">';

		if ( $previous_post ) {
			echo '<div class="pagination-previous"><a href="' .get_permalink( $previous_post->ID ). '"><span>Previous Chapter</span>' .$previous_post->post_title. '</a></div>';
		} else {
			echo '<div class="pagination-previous"><a href="' .get_post_type_archive_link( 'book-post' ). '"><span>Chapter Summaries</span>Book Title</a></div>';
			}

		if ( $next_post ) {
  			echo '<div class="pagination-next"><a href="' .get_permalink( $next_post->ID ). '"><span>Next Chapter</span>' .$next_post->post_title. '</a></div>';
		} else {
			echo '<div class="pagination-next"><a href="' .get_post_type_archive_link( 'book-post' ). '"><span>Chapter Summaries</span>Book Title</a></div>';
		}

		echo '</div>';

	}

}

style.css

/* Adjacent Pagination
--------------------------------------------- */

.pagination {
	clear: both;
	margin: 30px 0 0;
}

.adjacent-entry-pagination {
	border-top: 1px solid #999;
	border-bottom: 1px solid #999;
	font-family: Lora, Georgia, serif;
	margin: 60px 0 0;
}

.adjacent-entry-pagination .pagination-previous,
.adjacent-entry-pagination .pagination-next {
	display: block;
	float: left;
	padding: 15px 0;
	text-align: center;
	width: 50%;
}

.adjacent-entry-pagination .pagination-previous {
	border-right: 1px solid #999;
}

.adjacent-entry-pagination .pagination-next {
	float: right;
}

.adjacent-entry-pagination .pagination-previous a,
.adjacent-entry-pagination .pagination-next a {
	color: #454545;
	text-decoration: none;
}

.adjacent-entry-pagination .pagination-previous a:hover,
.adjacent-entry-pagination .pagination-previous a:focus,
.adjacent-entry-pagination .pagination-next a:hover,
.adjacent-entry-pagination .pagination-next a:focus {
	color: #4251ce;
	text-decoration: underline;
}

.adjacent-entry-pagination .pagination-previous span,
.adjacent-entry-pagination .pagination-next span {
	display: block;
	font-style: italic;
}

.adjacent-entry-pagination .pagination-previous span::before,
.adjacent-entry-pagination .pagination-next span::after,
.archive-pagination .pagination-previous a::before,
.archive-pagination .pagination-next a::after {
	content: "\f341";
	display: inline-block;
	-webkit-font-smoothing: antialiased;
	font: normal 18px/1 'dashicons';
	padding: 0 3px 3px 0;
	vertical-align: middle;
}

.adjacent-entry-pagination .pagination-next span::after {
	content: "\f345";
}