This is a metaguide - a highly selective set of links to pages on Othello strategy
Download WZebra, a computer program which can analyze your games and tell you where you went wrong
The standard beginners strategy guide (Colin Springer's translation of the FFO guide) comes with WZebra in English. You can get similar guides at VOG and Yahoo, or in Nederlands or Deutsch.
The book "Othello: Brief and Basic" is available from most national Othello federations. Online there are a number of useful sources.
George Ortiz's opening strategy also available in Deutsch
British Othello Federation newsletters