The Scholastic Dictionary of Spelling by Marvin Terban is a good reference material for students. It contains memory tricks for unpredictable words (ex: my adv ice is don't slip on the ice .) It also explains useful spelling rules for adding suffixes, forming plurals, etc. and has an age-appropriate spelling dictionary.
Upper-Grade Phonics (Grades 4-6) by Creative Teaching Press gives you plenty of spelling rules to learn and practice. (For example, this text explains when /shun/ at the end of the word is spelled sion and when it is spelled tion.)