Yes, this is actually quite common in a wide variety of contexts. A citation indicates the external source for a direct quote or paraphrase.
Throughout the first half of the novel, Strether has grown increasingly open and at ease in Europe (Henry James, "The Ambassadors", pp 34-40); this quotation demonstrates openness and ease.
Alternately a footnote can be used for the same purpose. Footnotes help keep the text free from too much external clutter:
Throughout the first half of the novel, Strether has grown increasingly open and at ease in Europe¹; this quotation demonstrates openness and ease.
and in the footnote:
- Henry James, The Ambassadors, pp 34-40.
Wikipedia uses this format in their articles, with the footnotes at the bottom of the article. However, an alternate method with hypertext documents like websites, is to include a link to the external source in the relevant text:
Throughout the first half of the novel, Strether has grown increasingly open and at ease in Europe; this quotation demonstrates openness and ease.
The exact style you should use depends on the context. Most professional publications have style guides with explicit instructions how to cite external sources. Other sites are less restrictive, and it may be a good idea follow what the most common style of other users.