Once your sticker pack has downloaded, you'll find it inside the Messages app by tapping that same App Store button you used earlier. (cultofmac.com)
This means you have your choice of stores, each named on the corresponding store button, to choose from. (pixelscroll.com)
To find free titles within the iBooks app, in the home screen tap on Store button in the top right corner, then select Top Charts section, and you'll see a heading saying «Free Books». (ebookfriendly.com)