I'll leave the explaining work to aohige then, since he can read Nihongo and has gone to where I've yet to explore (I have to rely on the Chinese version from Kadokawa Taiwan) =3
Btw, there're actually three spin-offs in addition to the main manga adaptation:
- "Log Horizon Gaiden - Honey Moon Logs" (ログ・ホライズン 外伝 Honey Moon Logs)
Featuring the Crescent Moon Alliance, led by the important supporting characters Mariel and Henrietta. The story appears to deviate from novel canon though.
- "Log Horizon Gaiden - Honcho Nyanta: Recipe of Happiness" (LOG HORIZON 外伝 -にゃん太班長・幸せのレシピ-).
The PoV character here is the Werecat Swashbuckler Nyanta (who will be voiced by the excellent Mr Nakata Jouji), and how, through cooking, he spreads happiness to the people around him =3. This manga appears to be set after volume four of the novel, but doesn't have any major spoilers atm afaik.
And of course, "Nishikaze no Ryodan" (ログ・ホライズン ～西風の旅団～) =3