We have 'other' as a type, but it includes both book and book child (chapter, usually). Other book / book child types are split out individually, these should be separated into separate types.
For example - this is registered as a book with book_type 'other':
http://api.crossref.org/works/10.5555/suffixtest
This is registered as a content_item (child of 'book') with component_type 'other' and also has type 'other' in the JSON:
http://api.crossref.org/works/10.4337/9781781001639.00001