Strike Witches Zero:1937 Fuso Sea Incident is a ongoing manga series being released in Nyantype magazine. It is illustrated by Ningen.
The story follows Sakamoto Mio as she receives training to become a Witch from Kitagou Fumika. The plot will later shift to the Fuso Sea Incident as the story progresses.
(Source: Strike Witches Wikia)

The Sky That Connects Us, often referred to as "Strike Witches 1.5", is a manga that was published in Nyantype magazine. Additionally, a Tankouban version with all eight chapters was published by Kadowaka at the end of September, 2010. It spans the time between the two seasons of the Anime. It covers the activities of the various members of the 501st after it was disbanded following the destruction of the Neuroi Hive above Gallia.
(Source: Strike Witches Wikia)

An expansion of the Strike Witches series, World Witches - Contrail of Witches details the short stories of various witches from areas of the European front of the Neuroi War.