keskikirja
Keskikirja is a Finnish term meaning "the middle book." It is used to refer to the central volume within a multi-volume work, especially in trilogies or series with three or more installments. The term is primarily descriptive and informal; it is not a formal bibliographic category in standard cataloging systems, and its usage can vary between publishers, editors, and readers.
In practical use, keskikirja denotes the book that sits between the opening volume and the concluding volume,
In some cases, keskikirja may appear as a working or stylistic label, or as the actual title