If you see a 
download icon on your book cover, or if you see the word ‘Download’ next to the book’s title, it means the book is available for download.
To download a book, just tap on its cover. Your eReader will connect to Wi Fi and download the book. When the download completes, just tap on the book title or cover to start reading.
In list view, you'll see the word "Download" beside the book title if it isn't downloaded onto your eReader:
In cover view, you'll see the download icon on the book cover:
Why do I need to download a book?
To save storage space on your eReader, your top five eBooks are automatically downloaded onto your eReader. The remaining of your eBooks are stored safely and securely online with Kobo.
If you prefer, you can download all your eBooks onto your eReader so that you don’t need to download them later. This can be useful if you’re going somewhere without access to Wi Fi and you want all your eBooks available on your eReader.
Read the steps here to learn how to download all your books onto your eReader.
Missing books?
If you purchased books and can't find it on your eReader, try the steps in these articles: