vigesimo-quarto

from The Century Dictionary.

• Said of a section of twenty-four pages in a book, on a sheet, or in form of type. Usually called twenty-four-mo.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Having twenty-four leaves to a sheet.
• noun A book composed of sheets each of which is folded into twenty-four leaves; hence, indicating more or less definitely a size of book so made; -- usually written 24mo, or 24°.