Log Horizon (ログ・ホライズン) is a novel by author
Mamare Touno.
The story revolves around 30,000 Japanese players who find themselves in a world mirroring the MMORPG (Masively Multipler Online Role Playing Game) "Elder Tale" they were playing, stuck in the bodies of their own character.
Shiroe is an Enchanter, a mage who specialize in supportive roles.
He and his friends seek to survive in a familiar, yet unfamiliar world of Seldecia.
The novel is published on the web first, and on book format afterwards.
There are six novels out, with the seventh still in web serialization.
Although the novel is considered light novel by many, technically due to the page count and its format, it's considered a "novel".
Anime adaptation started airing on 10/05. The thread can be found
here.
Web manga adaptation is currently serialized in Famitsu Comic Clear, with one volume out in publication.
Also a spin-off manga
Log Horizon - Nishikaze no Ryodan (ログ・ホライズン 〜西風の旅団〜) is currently serialized in digital manga magazine Age Premium, featuring stories of Soujirou Seta and his <s>harem</s> guild <Seifuu no Ryodan> (The West Wind Brigade).