Although I am a big Devilman fan, and have read a couple volumes of Amon, I am not 100% sure how it ties in with the Devilman continuity. So what I will do is summarize Devilman, and then review Amon. Beware, this review will contain Devilman spoilers, it would be pretty hard to write this without them. Devilman is the story of Fudou Akira, a timid, bullied teenager who is living with his friend Miki due to his parents being away. (They are archeologists.) One day, Akira's best friend, Asuka Ryo tells him he has something urgent to tell Akira and takes him to his house where he explains that demons once ruled the world but were frozen during the ice age. For a long time they were trapped, but now many of them have been freed. Ryo tells Akira that the only way to combat the demon threat is to unite with one. Furthermore, demons can only be controlled by a person pure of heart. Therefore, Akira must unite with a demon to become mankind's savior. With Ryo's help, Akira is able to unite with Amon (Hell's god of war.) and he becomes Devilman. OK, now fast forward to Amon. (Here come the Devilman spoilers.) Miki is dead. Akira's precious friend was killed by the very humans he was protecting. Ryo was actually satan - in human form. The trauma of Miki's death causes Akira to fade into himself and this allows Amon to take over. At the same time, it seems that satan has initiated the apocalypse. (There is this goup spreading from Japan throughout the world, melting/absorbing people.) satan works to bring the demon race under it's(?) (...satan is a hermaphrodite. I can't say "his" or "hers".) control as they run rampant, killing people. The human race, facing the demon threat has entered into martial law and chaos, frequently killing each other even. It seems the world is headed for destruction, and Akira's friends Dosu 6, Mericou-jin (I think I spelled that wrong.), and Miiko along with some other unidentified psychic figures, try to restore Akira's consciousness. While all of this goes on, Amon has only one thing in mind - killing satan. If you can tolerate the violence and demonic themes, Amon is a solid story delivered by the pen of the great Go Nagai with excellent art by Yuu Kinutani. It's not for everyone, but I recommend it to all who are confident they can handle it.

This series is a complete remake of the original Mao Dante, with new designs an a story slightly inspired by the anime of 2002. This time, the story spans longer, the character design is different, and all unanswered questions left when the original series was interrupted, are responded. This time, Mao Dante has a proper ending.
The story is divided in four parts: Kami Ryaku Hen, Gen Ma Hen, Mado Hen and Kami Ma Taisen Hen. (mangaupdates.com)

After the world ended in the original Devilman, Akira Fudo is in hell, seeking the soul of his beloved Miki while at the same time Doctor Hell is beseeching the underworld for power in order to get his revenge upon the hated Mazinger. Akira sets in motion a chain of events that earn the enmity of Hades himself, who'll stop at nothing to avenge the slights Akira commits against him...

Spring, 1991. Mysterious gold metal balls fall from space... The balls were too small to be sensed by humans, so they were scattered across the entire earth without incident!

Included one-shot:
Zipang

A short manga version of the UFO Robo Grendizer anime. Released along the anime series, UFO Robot Grendizer follows the adventures of Duke Fleed, the prince of Planet Fleed, who, disguised as Daisuke, a farm worker, fights against the forces that destroyed his planet, the Vegan Empire, also the creators of Grendizer that Duke stole to fight them. While not originaly considered part of the Mazinger series, Koji Kabuto and the Mazinger mechas appear in this series, making it sort of a sequel. Grendizer introduced the concept of a transformable robot and a space mecha, concepts that were later taken into other mecha series. It was extremely popular in Japan, Canada, France, Italy, and the Arab world and has a strong following in those countries even today.

A shocking and violent look at a post apocalyptic future. Many different stories centre around an array of characters dealing with life in this desolate world. All the stories feature a huge man named Jack who is in an ongoing battle with a villain named Slum King.
(Source: ANN)Note: This is kanzenban version which consists of: 7 volumes (published by Kodansha) and 31 volumes (published by Nihon Bungeisha) merged into 18 volumes.