on Jane Friedman site:
When an author submits a book proposal to a university press, in a best-case scenario the acquiring editor will think the project is promising and want to go ahead with peer review of the proposal and some or all of the book manuscript. At some publishers, acquisitions editors present projects they are excited about to other press staff and are then approved by an internal committee to proceed with peer review. At other presses, editors can proceed with peer review at their own discretion. Peer review is a practice that distinguishes scholarly presses from other types of publishers, so it’s key for authors to understand how it works and what expectations will fall to them as a result.
If you make it to the peer review stage, your editor will ask you to provide the materials they need for review. Many presses will move forward to peer review with just a proposal and sample chapter or two; some presses prefer to wait on peer review until the author provides a full or nearly complete manuscript, especially for first-time authors.