A secondary source is one that uses primary sources as its evidence. Most academic journal articles, books, etc analyze primary sources to answer their questions.
Tertiary ("third-level") sources use secondary sources as their primary evidence. These usually include things like encyclopedia and dictionary articles.
Something like a documentary, or a video game, or the history papers you write could theoretically be either, or be more one than the other. If you really need to tell for sure, you need to examine the sources they use. However, in the real world the distinction usually isn't crucial.
This page will show some of the ways to find and access non-primary sources at CFL.
CFL has over a million titles available, either physically in the library or available online, and many millions more available through Hathi Trust. Most of the books we have published in the last twenty years are available online rather than in the stacks. If you're looking for a particular book, you should first see if it comes up in our library catalog on the main library page.
If you have the LibKey Nomad browser extension installed, and configured to the University of North Dakota, it will flag when a book you find on Amazon is available as an ebook, and provide a link.
The following databases have many academic history books:
Finding a good history book can be daunting. There are many sources of book reviews, either very short (like from Choice) or long and address their relationship to the academic discussion. The following are all good sources of book reviews for historians: