Vocabulary is key
A good vocabulary is the foundation for your information search
If you search with the words you have on your mind right here and right now, you can very well risk getting some very one-sided results. Find more words for the same concept to get different takes on the topic and perhaps get to a more academic level. Look through the table of contents and see what technical terms the author uses.
Scholarly books versus practical books
How do you know that the book/books you have found for your assignment is actually a scholarly book and not just a practical book. Watch the video and learn how to differentiate between the different kinds of sources.
Video: Scholarly books (by Mc Masters University)
Find both pros and cons
It is important that you find sources that speak FOR your theory and sources that go in a different direction. Nothing can stand alone and to illuminate a given theory; as to describe a topic, you need to find what speaks FOR your theory / your argumentation - but also what speaks against. So Pros and cons.
Some databases have subject term lists to help you do better (or smarter) searches in their databases. They will do an inventory over the subject terms used and you can find the lists in the database. EBSCO is one of the databases where you can find a subject term list.