Non-peer reviewed sources, which are still high quality and credible, can be very useful when looking for information about your mathematician (like biographical details and overviews of their work and impact).
When someone talks about a peer reviewed article, they usually mean a research article published in an academic journal that conducts peer review. These journals will publish other, non-research materials that do not go through the same rigorous peer review process because these materials do not contain new scientific work that needs to be evaluated. These are things like: letters to the editor, book reviews, and trends and highlights relating to the journal's subject area. These materials are still written by experts in the field for other experts and the journal's editors must review for accuracy and relevance.
Books that are written by experts in a field and edited by other experts can be excellent sources of quality academic information. These can be found in the Library's catalogue and ebook collections.