If you're looking for manga similar to Steins;Gate: Egoistic Poriomania, you might like these titles.
The story is told by Albert N. Wilmarth, an instructor of literature at Miskatonic University in Arkham, Massachusetts. When local newspapers report strange things seen floating in rivers during a historic Vermont flood, Wilmarth becomes embroiled in a controversy about the reality and significance of the sightings, though he sides with the skeptics, blaming old legends about monsters living in uninhabited hills who abduct people who venture too close to their territory.
When the strange twins Wilbur Whateley and his invisible brother, born out of forbidden black magic, attempt to open the gates to a dark world using the Necronomicon grimoire... Miskatonic University's head librarian Henry Armitage and his colleagues are here to stand in their way!
Kuro Daijuuji is a poor detective living in Arkham City. One day, he was requested by Ruri Hado of Hado Financial Group, to search for a magic book. While he initially refused, Ruri offered him a large sum of money upon completion of her request, in which bribed Kuro to accept. As Kuro searches for the book, he unexpectedly runs into Al, a pretty girl that is actually a powerful grimoire. They forge a contract with each other, bestowing Kuro with powerful magic. Soon afterwards, Al also activates Demonbane, a deus machina owned by the Hado Financial Group, to combat the mechanical menace from the Black Lodge. With this, the war between the Hado Financial Group and the Black Lodge begins....