Notable Reads and Things:

English-based:

• Kyoukai Senjou no Horizon Wiki
• Baka Tsuki: Kyoukai Senjou no Horizon Light Novel Translation [I highly recommend perusing the glossary, history, and character pages]
• HimaHimaSeijin Scans [Working on scanlation of the Horizon manga]
• Translations on the Middle of Nowhere [Working on scanlation of the Horako-san 4koma]
• AnimeSuki Forum: Horizon Novel Discussion
• AnimeSuki Forum: Horizon Anime Discussion

Japanese-based:

• Official Anime Website
• Official Anime Twitter
• Kawakami Minoru’s Light Novel Website
• Kawakami Minoru’s Twitter
• 境界線上のホライゾンまとめ＠ウィキ
• 境界線上のホライゾン個人的まとめ@ウィキ
• 境界線上のホライゾン まとめウィキ
• 川上稔作品大全 (chara profiles, glossaries, and “How to Appreciate Kawakami Minoru’s Works”)