The history of English is the history of its spelling and there is no other comprehensive and highly readable guide to the history of spelling than David Crystal’s Spell It Out: the Singular History of English Spelling (Profile Books, 2012).
For more on English dialects this blog has a wealth of information Dialect blog.
If you’re interested in linguistics and the history of language, NativLang is a really interesting YouTube channel.
For English learners
Sushi spell is a fun game to play and practice making words (you need Flash player, and audio to listen to the music!)
For pronunciation practice the app Say It helps you see and correct your pronunciation. It was developed by an British English teacher and Oxford University Press, and if you’ve ever used English File you’ll recognise the cue words. For iPad and iPhone here.
Strategies to improve your spelling on Spellzone
Lots of spelling rules and patterns on How to Spell
If you’re teaching spelling to English learners you need Teaching Spelling by Johanna Stirling (Lulu 2011) It provides an overview of the different spelling systems employed in English, and bank of activities to use in class, both to assess learners’ ability and to help them recognise spelling rules and learn strategies to remember spellings.
For teachers in Japan the Language Teaching Professional group provides teacher training, workshops, expos and a book service.
To learn about the problems that English learners might encounter related to their native language, including pronunciation, grammar and vocabulary, the classic book is Learner English edited by Michael Swan and Bernard Smith.