Using the Genericons icon font for WordPress dashboard icons

admin-style.css

#menu-posts-book-review .wp-menu-image {
  background: none!important;
}
#menu-posts-book-review .wp-menu-image:before {
	font-family: 'Genericons'!important; /* this assumes you've loaded Genericons in your admin */
	content: '\f444'; /* replace this with your icon */
	font-size: 20px;
	position: relative;
	left: 5px; /* positions the icon */
	top: 4px; /* positions the icon */
	text-shadow: 1px 1px 0px #eee;
	color: #aaa;
}
#menu-posts-book-review:hover .wp-menu-image:before, .wp-has-current-submenu#menu-posts-book-review .wp-menu-image:before {
	color: #21759b;
}
.wp-has-current-submenu#menu-posts-book-review .wp-menu-image:before {
	text-shadow: 1px 1px 0px #555;
}
div#icon-edit.icon32-posts-book-review {
	background-image: none;
}
div#icon-edit.icon32-posts-book-review:after {
	font-family: 'Genericons'!important;
	content: '\f444'; /* replace this with your icon */
	color: #555;
	font-size: 32px;
	position: relative;
	top: -5px; /* positions the icon */
	left: 5px; /* positions the icon */
}

/* this adds support for MP6 & future WP (3.7/8, whichever gets the dashboard css update) */
/* this is largely thanks to: https://github.com/Gizburdt/Wordpress-Cuztom-Helper/issues/152#issuecomment-17490472 */
.mp6 #adminmenu #menu-posts-book-review div.wp-menu-image:before {
	font-family: 'Genericons'!important;
	content: '\f444'; /* replace this with your icon */
	left: 0; /* resets the positioning for MP6/future WP */
	top: 0; /* resets the positioning for MP6/future WP */
	text-shadow: none;
}