On both the app and website, you will see a magnifying glass icon in the upper right-hand corner, and you can use that to search for both specific authors, titles, and keywords.
Website
On the website, you can go to the "Free Books" tab at the top and select a genre, which also lets you sort by the latest or most popular stories and select filters on the right side.
App
On the Discover page in the mobile app, you can find tabs for both New and Popular books, along with the Explore Genres button in the upper lefthand corner, which lets you navigate through popular tags and topics in the respective genre (along with filters for popular, new, and recently updated stories).