Unfortunately, it doesn't work:
Code:
Setting extended header data: 501 - EBOK
EXTH setting data: 501 - 501 - EBOK - 0x45424f4b
EXTH add: 501 - 501 - EBOK
GETSTRING: Author - Twain, Mark
CoverOffset - not printable
ThumbOffset - not printable
CDEContentType - EBOK
but the book still appears under "Documents" and not "Books". Interestingly, AZW3 books (converted with calibre) appear under "Books".