Scholarly sources are written by highly-qualified researchers and have a thorough publication process, which usually involves peer-reviewing and an extensive list of references at the end of the text. Scholarly sources often have a specific audience in mind, most likely other experts in the particular field of study. Examples of scholarly sources include books and academic journals written by scholars and experts.
(from Purdue OWL's "Evaluating Sources: Where to Begin")
Peer-review is a publication process where research articles written by experts in academic or professional fields are submitted to reviewers who are also experts in the field.
To locate peer-reviewed articles, Summon and most library databases will provide a peer-review limiter that limits results to journals that publish research articles with a peer-review process. Letters, commentaries, news reports, obituaries, book reviews are generally not peer-reviewed, even if the journal uses a peer-review process for research articles.
(from Oberlin College Libraries' FAQs)
Scholarly books are written for scholars/researchers in the author's field and are typically intended to share research findings and contribute to the ongoing scholarly conversation. Scholarly books consist of the following:
(from UTSA Libraries' "Scholarly Books")