Riichi Mahjong Wiki — probably one of the best sources of riichi mahjong rules in english.
Riichi Mahjong Strategy Books written by Daina Chiba. This book is freely available. Highly recommended.
Riichi seminar — Daina Chiba seminar. For more advanced players.
USPML — many materials about riichi mahjong.
Osamuko’s Mahjong Blog — many articles about riichi mahjong, events, competitions, etc.