Core Skill Workbooks by Steck Vaughn help students build skills necessary to succeed in school and provide meaningful practice for a variety of subjects and grade levels. They are a great resource for additional skill practice as well as a tool to increase scores on standardized tests. Answers included. Core Skills Workbooks are a Houghton Mifflin Harcourt School Supply Best Seller! Available for subjects K-8: Grammar, Language Arts, Writing, Phonics, Reading Comprehension, Spelling, Math, Algebra, Social Studies, Science and Test Preparation.