Short work that examines a topic within a broader topic. In this case, More's Utopia in the context of other Utopian writings. These provide some background information, but are framed by a larger argument.
Short, argumentative, narrowly focused. They only give information directly related to their thesis, so they bad starting points. They are extremely helpful, though, once you have a thesis.
Extremely narrow, with the focus of an article but the length of a book. Typically you don't need one of these unless it is recent and directly related to your topic.
Why it matters?
When you start a topic, consider:
Is this a book-length topic or an article-length topic?