Attack on Titan: Lost Girls started out as a pair of small visual novels that were packed in with Japanese Blu-ray releases of the Attack on Titan anime. The story was then adapted into a novel by Hiroshi Seko, followed by Ryōsuke Fuji’s manga. Next up is an anime adaptation, which will arrive in the form of an OAD included with volumes 24, 25, and 26 of Hajime Isayama’s manga.
Volume 24 hits shelves in Japan on December 8, followed by volume 25 on April 9, 2018 and volume 26 on August 9, 2018.
Vertical released the novel in English and Kodansha released the manga. Here’s how the former describes the story:
LOST GIRLS tells of the times and spaces in between the plot points, through the eyes and ears of the saga’s toughest—but more taciturn— heroines. What they thought, and felt, and did on and off stage come to the fore via a medium distinctly suited to conveying interiority: language.
Source: Moetron via Crunchyroll