Secondary sources are theoretical or critical texts which are part of the professional wisdom on or related to your primary object of study. These are texts representing individual contributions to the intellectual exchange surrounding the phenomena you've chosen to examine in your own work, the critical conversation you're joining and complicating with your own contribution. Different genres of secondary source you're likely to use for the third essay are: dictionaries and encyclopedias; monographs (academic or theoretical books devoted to a particular topic); articles published in peer-reviewed scholarly journals and other periodicals, and, possibly, websites and blogs.
One big part of producing college-level original work is learning how to gauge the authority of secondary source material or how to distinguish secondary source material which is eligible to inform academic work from material which is ineligible to inform that kind of work, simply because it has not been sufficiently scrutinized or subjected to sufficient critical or editorial rigor.
Academic monographs. Monographs are books addressing themselves to a particular topic. They can be books written by a single author or edited volumes of contributions by multiple authors. In all cases before they are published manuscripts are vetted by multiple readers qualified to judge the comprehensiveness, currency, objectivity, originality and validity of work on the given topic. When trying to determine whether or not a book qualifies as "academic," one thing to look at is the publisher. If the book is published by a University Press, you can consider it an eligible secondary source. Another thing to look at is bibliographic documentation. Research publications document their own source material, so a book you're using as one of your secondary sources should have extensive citations and/or a bibliography.
Peer-reviewed journals. Academic journals using the peer-review editorial and selection process circulate all submissions to multiple recognized academic experts in the relevant field in order to determine whether a submission should be published as-is, whether ammendements or revisions are required, or whether the submission should be rejected.
Newspapers and Magazines. Professional journalism is still a high-barrier publishing arena; journalists are often expected to have advanced academic credentials in order to qualify to write on particular topics. In addition to the institutional mecchanisms of fact-checking and copy-editing, the work of journalists writing for widely circulated publications (e.g. the New York Times, the Wall St. Journal, Atlantic Monthly and The New Yorker) is exposed to the millions of critical eyes of a vast readership which inevitably includes many highly educated people qualified to judge the validity and objectivity of information on a given topic. This readership ensures a critical rigor somewhat akin to that provided by peer review and other editorial mechanisms in academic publishing.
Blogs and Websites. The Internet is, of course, a very low-barrier publishing arena. It's pretty much the case that anyone can publish anything in cyberspace, and this means that the only people who can feel comfortable using Web publications as source material for academic work are people expert enough in the relevant subject matter to independently gauge the comprehensiveness, currency, objectivity and overall worthiness of work in that area. That said, more and more academics and other experts, as well as academic institutions and their divisions, are maintaining their own websites and blogs and increasingly these are being cited as sources in academic work. If you would like to use a website or blog as a secondary source in your work, it's important that you learn enough about it and its author to determine whether it qualifies as a legitimate secondary source. Like with monographs, one of the first indications of research quality to look for in a blog is documentation. The better documented a blog post is, i.e. the more the author refers to and provides citations for sources consulted, the more likely it is that the blog qualifies as a secondary source for academic research.
A crucial practice in academic work is treating all source material critically, irrespective of the publishing venue or authors' qualifications. Valuable secondary source material can come from anywhere, and no piece of research or writing is without its limitations and biases. It's important, however, to maintain an awareness of the extent to which various sources have to withstand scrutiny - and whose scrutiny – in order to be published when selecting sources and deciding how to incorporate and weigh them in the larger sphere of discourse that contextualizes your own work.
Some documents can serve either as primary or secondary sources depending on how they're being used in the given context. It's important to be aware of how you're treating a particular source, i.e. as primary or secondary source material, when formulating your selection criteria. A newspaper or magazine article, if determined to be sufficiently authoritative, balanced and well researched, might be used as part of the analytical discourse which you engage and extend in your paper. In this case, the article is a secondary source. A newspaper or magazine article which itself instantiates a particular bias or which is itself a document of the social/political/cultural phenomenon you're studying is a primary source. The distinction is an important one to remain aware of, as authoritativeness, transparent documentation of source material, and objectivity are not required of primary sources, but are requirements for secondary-source eligibility.