The type of the release group of audiobooks (album, single, EP, broadcast, other) is not clear. A lot of release groups are entered as "other", slightly few as "album".
https://musicbrainz.org/doc/Release_Group/Type says this about the primary types:
Album: An album, perhaps better defined as a "Long Play" (LP) release, generally consists of previously unreleased material (unless this type is combined with secondary types which change that, such as "Compilation").
Other: Any release that does not fit or can't decisively be placed in any of the categories above.
http://musicbrainz.org/doc/Style/Release_Group and http://musicbrainz.org/doc/Style/Specific_types_of_releases/Audiobook don't mention the primary type.
IMHO audiobooks match the description of "album", but I have no problems with the "other" type.
So what primary type should audiobooks have?