Gekiga, Japanese for "dramatic pictures," was a response by artists who wanted to create something more serious than the kid-oriented manga. Eventually, Tezuka picked up influences of this style and modified his own work. Because of Tezuka's place in the mainstream, this style became wide-spread, kicking off the golden age of manga (1970s through 1980s). As shonen manga became more commercialized, however, it began to lose the gekiga influence, and manga in the gekiga style have become alternative and under-ground.
Resources in the Ohio State Library
Gekiga Hittorā by Muzuki Shigeru
Gekiga Mō Takutō den by Fujiko Fujio
Gekiga Furansu Kakumei to Naporeon : zōkan Rekishi to jinbutsu by Rarūsu hensan ; Takikawa Kōyō yaku