small-format paperback; pocket paperback
Refers to the compact, affordable paperback format common in Japan, typically A6 size, often used for reprints of novels and light novels.
通勤中に文庫本を読むのが好きです。
I like reading pocket paperbacks during my commute.
This bestselling novel was finally released as a pocket paperback.
It's cheaper to read it in the paperback edition than the hardcover.