Book chapters can be of various types. Two of these types are scientific contributions to proceedings of conferences or contributions to a thematic collection of papers. For both types peer review is possible, and such "chapters" may have similar content and the same quality as papers in journals. When writing a systematic review, I would not exclude such book chapters. But you should reed the instructions of the journal for which the review will be written.

Many a time people dont do this but it is good to do a review before publication, although editors do the required checks. Editors can give the reviewers to follow certain guidelines/instructions. In this way more quality improvement will be there.

It depends on your research question and protocol. I usually follow JBI (Joanna Briggs Institute) manual and CRD (Centre for Reviews and Dissemination) for reference.

A simple rule of thumb: If the article/chapter is peer reviewed, you can use it. If it is not peer review, it should not be included.

You can also read:

Shades of Grey: Guidelines for Working with the Grey Literature in Systematic Reviews for Management and Organizational Studies

Richard J. Adams, Palie Smart, Anne Sigismund Huff; https://doi.org/10.1111/ijmr.12102