Reading your sources means engaging in a conversation with other scholars. It might help to imagine authors as "talking" to you about their research. The authors of your secondary sources have information that will help you understand your chosen topic — data that will clarify some issues you may have, and data that might lead you to a research question or problem.
Booth, Colomb, and Williams suggest that you read your sources to "test and support" your answers to questions you may have about your topic, including data and arguments that may "distort ... or even contradict" what you feel strongly about your topic (84).
I would suggest that you first read and understand each source before taking down notes (you can highlight or underline some passages, if it will help you understand the text). It's important that you have a good grasp of the author's main point so that when you do take down notes (or make marginal comments in your photocopied material), you know the context of the idea you might be citing in your paper (and don't forget to record the publication data for each source).
Booth, Colomb, and Williams also imply that reading means re-reading each source, especially if the information the text contains (88):
They suggest several ways of reading each source: reading for "creative agreement" to focus a topic, reading for "creative disagreement," and reading "to plan your argument" (88-94).
In reading for "creative agreement," the authors suggest that you engage with your source by (88-89):
Booth, Colomb, and Williams suggest reading your source for "creative disagreement" by imagining the opposite of what are claimed in the text (89-91).
And they suggest reading your source "to plan your argument" by (92-94):
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