While much of the content of your course may be documents you've created in other applications and then uploaded to D2L, when you are setting up a reading list with links to resources that are external to D2L (e.g. a journal article from a publisher's web site), you will need to edit the links in the D2L HTML editor.
Step 1) In the HTML editor, choose the 'Insert Stuff' menu and then
Step 2) 'Insert Link'.
Next enter the proxied persistent URL for the article (or chapter, etc.)
Next enter the text students will click to access the article. Click 'Insert'.