An excellent guide to english spelling rules can be found at the davidappleyard.com World of English Hub.
The other resources on the site include:
- English User's Bookshelf
- Dictionary Bank
- Grammar Glossary
- Article Usage
- Tense Usage
- Irregular Verb Chart
- Irregular Verb Patterns
- Pronunciation
- Punctuation
- Spelling
- Territory-Related Words
- Words Often Confused
- American-British Glossary
- British-American Glossary
- Phonesthemic Initials