When writing a thesis/OLP can I also cite books or am I restricted to peer-reviewed articles?
The type of literature you cite and where you find it depends on the questions you are answering.
There are topics that are simply not covered in the scholarly press, and books, magazines, newspapers, blogs, videos or even twitter feeds might offer more current and relevant information.
Your instructor or supervisor is the best person to advise you on the credibility of resources. They are the subject experts in your field and can advise on the suitability of specific sources.