1/ the dir name can not contain ".-", in case of module it must not contain "_"
2/ the dir name should match the language file name
3/ there should not be old style en_utf8 dir
4/ there should not be the postgresql.sql and mysql.sql files
5/ the version number if present must be above 2010000 (already there)
6/ all letters in dir name must be lowercase

I have witnessed many false alarms from people that named the book folder incorrectly, the worst part is if the tables are created from the wrong plugin, then you can not install the new plugin any more.