Desktop App. From the Mac or Windows Bookshelf desktop app, hover your cursor over the title you want to download and click Download Book while connected to the internet. Once the book has been downloaded, a small checkmark icon will appear in the top-right corner of the title on your library. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.