paperback edition; bunko edition
Refers to a compact, affordable paperback format common in Japan, often part of a series like 文春文庫 or 角川文庫.
The paperback edition of this novel will be released next month.
新書版 is a slightly larger paperback format (typically 173×105 mm) often used for non-fiction, whereas 文庫版 is smaller (typically 148×105 mm) and used for both fiction and non-fiction.
Compound of 文庫 (bunko, 'library' or 'book collection') and 版 (han, 'edition'). The term originated from the practice of publishing compact, affordable reprints for personal libraries.