We supplement our online curriculum with all kinds of workbooks. Recently we have given "240 Vocabulary Words Kids Need to Know" a go and are quite happy with it.
Description on back of workbook:
Build word power with these 24 ready-to-reproduce, 3 page lessons. Each lesson includes research-based activities that build on students prior knowledge and gives them multiple encounters with the new words so they really remember them. Lesson topics include prefixes, suffixes, Latin and Greek roots, homophones, antonyms, synonyms, and lots more. Watch reading skills and test scores soar!
