Great news! A READ NOW button means that any NetGalley member, when signed into their account, will be instantly approved for that book. Just press the Read Now button and you'll see the book added to your Dashboard, and on your Shelf, too.
Some publishers opt to enable the READ NOW feature for books that they want to promote widely, or for special content like catalogs and samplers. You can see a full list of Read Now books 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 books in a specific genre.