Secondary sources fall into several different categories:
Scholarly historical sources
These are sources produced by professional historians who have had advanced training in the discipline of history. The works that they produce will almost always include documentation, usually in the form of footnotes or endnotes, or, much less frequently, parenthetical citations. Their works are often, though not exclusively, published by university presses. In general, these will be the most useful and reliable of the secondary works that you will consult. As a general rule any book that includes extensive citations (especially if citations lead to academic or primary sources) is safe to use as a reference in an essay. Although if you are in doubt you should always ask your Professor.
Within this category you will find three fundamental kinds of works:
(I) Journal articles. These are articles that you will find in scholarly journals/periodicals, such as the American Historical Review or the English Historical Review. They will be of two types.
Research articles present the research findings of historians on given issues. The documentation in them will consist mostly of primary sources.
Review articles are reviews of the historical literature in a given field of history. To produce articles like this, historians read a large number of books on a given topic, and then attempt to summarise the latest trends in scholarship on the issue. They may also point out areas they feel historians have not yet sufficiently examined. The documentation in these articles will usually consist of citations of the secondary sources that the author is discussing, which are extremely helpful for trying to find sources on a particular topic, although the more recent they are of course the more relevant they will be.
Please note that many research articles include incorporate elements of review articles into them, usually in the first few paragraphs.
(2) Monographs. These are books that are like very long journal articles. These usually focus on one specific historical question or issue, and deal with it in great detail.
(3) Scholarly syntheses. These are broader historical works that attempt to summarise and interpret the more detailed works that historians have been producing in journals and monographs. Syntheses will usually include both primary and secondary sources in their documentation.
Other scholarly sources
Naturally since history is such a broad genre it overlaps with many other fields of academic study such as anthropology and political science. Of course you should not hesitate to use resources from other academic fields of study in your research, provided that they are from scholarly sources. In fact professional historian do so all the time.
Popular and journalistic sources
These book-length or article-length works are written for a more general reading audience. Sometimes they can be very useful, but, overall, they tend to be more uneven in quality, and they usually lack the documentation that would make them really useful for your purposes. Do not use popular resources such as newspapers and magazine articles without receiving prior approval from your Professor, as these are not accepted scholarly resources. This doesn't mean that you should hesitate to use them as a "jumping off" point for your research however, unfortunately the same doesn't always hold true for online resources.
The World Wide Web
A preponderance of the "historical" material posted on the WWW is worthless. Bear this in mind: To publish a scholarly article or book, historians must go through a process of research, writing, rigorous editing, and checking facts. Other historians and editors then scrutinise their work writers of popular and journalistic history also must go through an editing process. But material can be posted on the World Wide Web by anyone who can type. Unfortunately since many websites undergo almost no scrutiny whatsoever and are accepted at face value, many websites cite/utilize articles which were wrong in the first place, to produce a synthesis of misinformation.
Moreover the purpose of citations is to allow whomever is reading an article to trace the authors information back to its "starting point". The way in which websites are maintained can make citations largely worthless because after a website changes (or goes offline) it is usually impossible to find what was previously present at a web address meaning that references to online sources are often citing something that is no longer present. Many academic journals are over a century old and their citations written a century ago can still be useful to a modern reader, but as things currently stand it is extremely hard to believe that this will also be the case with online sources.
Electronic Journals do not have many of the same problems as other online sources because they are usually exact copies of what was published in their paper version (see journal articles above), and they are not subject to changes and "updates" over time. The same is the case with E-books.