library/library_website/views/website_assets.xml

<?xml version="1.0" encoding="UTF-8" ?>
<odoo>
    <template id="assets_frontend"
              name="library_website_assets"
              inherit_id="website.assets_frontend">
        <xpath expr="." position="inside">
            <link rel="stylesheet" type="text/css" href="/library_website/static/src/css/library.css"/>
        </xpath>
    </template>
</odoo>