Gray Welsh Cob by bambe1964 Attribution-NoDerivs Licence
Conference proceedings and textbooks are often good sources to find out what experts are thinking in a discipline. Some conference proceedings also provide the early reports of new scientific findings that might be pertinent to your work. However you need to cite non-peer reviewed literature sparingly, if at all.
One time when you may wish to cite textbooks or conference proceedings is when you need to support statements about another author’s opinion. In that case make it clear that you are citing opinion and not empirical published observations. Another time is when the research is so new it is only available in a conference proceedings abstract. In this case be very sure that you have checked thoroughly for the peer-reviewed version of the publication and you should only be citing very recent conference abstracts in this case (less than 2 years old).
Remember to distinguish the peer-reviewed and non-peer-reviewed literature carefully in your review. The reader needs to be able to tell if you are citing non-peer reviewed material.