Although books are the dominant form of scholarship in the humanities, articles are valuable, especially those appearing in scholarly journals.
Some assignments require one to limit one's search to scholarly or peer-reviewed articles. "Peer-reviewed," "scholarly," "academic,"and "refereed" are often used interchangeably to describe manuscripts that underwent expert review before publication.
Some databases allow one you to limit your search to scholarly journals.
Most searchers rely exclusively on keywords to retrieve records in online databases. If you wish to retrieve more precise results, try searching by controlled vocabulary for your topic, often referred to as "subjects" or "descriptors."
Spanish surnames can be tricky. Urrea, Luis Alberto will get the best results as a subject or author, and perhaps more than Luis Alberto Urrea as a keyword search. However, many surnames are compound (e.g., Garcia Marquez, Gabriel).
Not all books contain a unified narrative from the beginning to the end of the text. Some are collections of essays, with each chapter having a unique title and author(s). These works share features with both books and journals. Like a book—unlike a journal—there is more lag time before publication. However, like a journal—unlike a book—catalogs do not trace the titles and authors of every chapter and researchers must use databases and indexes to search for chapters by a particular author or on a specific subject.
Book chapters are particularly common in literature, and as with journal articles, are well represented in MLA.
If you have difficulty attempting to track down an essay or a chapter in a book, I am happy to help.