Great news! A READ NOW button means that any NetGalley member, when signed into their account, will be instantly approved for that title. Just press the Read Now button and you'll see the title added to your Dashboard, and on your Shelf, too.
Some publishers opt to enable the READ NOW feature for titles that they wish to promote widely, or for special content like catalogs and samplers. You can see a full list of Read Now titles in the catalog by going to Find Titles, and then under Browse click Read Now. You can also click on any Category page to find Read Now titles in a specific genre.