A snippet is actually just a QWeb view that gets injected in the Insert blocks bar, which is defined by a QWeb view itself:

1. Add a file called views/snippets.xml:

<?xml version="1.0" encoding="UTF-8"?> 
<odoo> 
    <template id="book_snippet" inherit_id="website.snippets"> 
        <!-- points 2, 3 go here /--> 
    </template> 
</odoo> 

2. Add a view for your snippet:

<xpath expr="//div[@id='snippet_feature']/div[hasclass('o_panel_body')]" position="inside"> 
    <div> 
        <div class="oe_snippet_thumbnail"> 
            <div style="background: white; box-shadow:none"                             
                 class="oe_snippet_thumbnail_img"> 
                <i class="fa fa-book fa-5x text-muted" /> 
            </div> 
            <span class="oe_snippet_thumbnail_title">
                                  Latest books
            </span> 
        </div> 
        <div class="oe_snippet_body book_snippet"> 
            <section class="container"> 
                <h2>Latest books</h2> 
                <table class="table"> 
                    <tr> 
                        <th>Name</th> 
                        <th>Release date</th> 
                    </tr> 
                </table> 
            </section> 
        </div> 
    </div> 
</xpath> 

3. Add options:

<xpath expr="//div[@id='snippet_options']" position="inside"> 
    <div data-selector=".book_snippet">
        <li class="dropdown-submenu"> 
            <a href="#">Show books</a> 
            <ul class="dropdown-menu"> 
                <li data-select-class="book_snippet-show3"> 
                    <a>3</a> 
                </li> 
                <li data-select-class="book_snippet-show5"> 
                    <a>5</a> 
                </li> 
                <li data-select-class="book_snippet-show10"> 
                    <a>10</a> 
                </li> 
                <li data-select-class="book_snippet-show15"> 
                    <a>15</a> 
                </li> 
            </ul> 
        </li> 
    </div> 
    <div data-selector=".book_snippet table"> 
        <li class="dropdown-submenu"> 
            <a href="#">Table style</a> 
            <ul class="dropdown-menu"> 
                <li data-toggle-class="table-striped"> 
                    <a>Striped</a></li> 
                <li data-toggle-class="table-bordered"> 
                    <a>Bordered</a></li> 
                <li data-toggle-class="table-condensed"> 
                    <a>Condensed</a> 
                </li> 
            </ul> 
        </li> 
    </div> 
</xpath>

4. Add JavaScript code to populate our snippet:

odoo.define('r3_snippets', function(require)
{
    "use strict";
    var rpc = require('web.rpc'),
        Animation = require('website.content.snippets.animation');

    Animation.registry.book_snippet = Animation.Class.extend({
        selector: '.book_snippet',
        start: function()
        {
            var self = this,
                number = 3;
            _.each(this.$el.attr('class').split(/s+/), function(cls)
            {
                if(cls.indexOf('book_snippet-show') == 0)
                {
                    number = parseInt(cls.substring(
                                          'book_snippet-show'.length));
                }
            });
            this.$el.find('td').parents('tr').remove();
            rpc.query({
                model: 'library.book',
                method: 'search_read',
                domain: [],
                fields: ['name', 'date_release'],
                sortBy: ['date_release desc'],
                limit: number,
            })
            .then(function(data)
            {
                var $table = self.$el.find('table');
                _.each(data, function(book)
                {
                    $table.append(
                        jQuery('<tr />')
                        .append(
                            jQuery('<td />').text(book.name),
                            jQuery('<td />').text(book.date_release)
                        )
                    );
                })
            });
        },
    });
});

After updating your module, you're offered a snippet called Latest books, which shows a configurable amount of books in a list, ordered by their publication date.