An abstract is a short summary that highlights the main points of a book, an essay, or a chapter in a book. Unlike an outline, which tries to encompass all the points in a work, organizing them by importance, an abstract is shorter and provides a quicker overview.
If you look at what eNotes has to say about Melville's The Confidence Man, you will note that the first summary, of only eight paragraphs, is shorter than the second, more extended summary, that delves in greater depth into the details of the novel. The first summary, though not short enough to function as an abstract, is meant to offer you an overview of the book to provide a context for the second summary.
To create an abstract, which usually is one paragraph long (it can be a relatively long paragraph, if need be), you could start by pulling the main topics from each of eNotes's eight paragraphs of initial summary. You would end up with a document that would look somewhat like what follows (you would want to finesse it to meet your own needs):
The Confidence Man takes place on April Fools' Day on a Mississippi steamship. The main character, the Confidence Man, a con man, appears in numerous disguises during this single day. Little happens during the novel in terms of action: Conrad focuses on conversations between the con man and his victims, making this a novel of ideas rather than a plot driven story. The Confidence Man's work illustrates that people are driven by self interest and that he only needs to find their weak points to be able to con them. The con man uses a number of disguises, such pretending to be a mute, posing as a disabled black man, dressing as the president of a Coal Company, and pretending to be an herb doctor in order to deceive people. He usually can successfully trick his victims, but makes little money doing so. The end of the book, in which the Confidence Man leads away a Bible-reading old man who has been conned by an innocent-looking young man, is left open to interpretation.