After First Language Lessons, Level 4, we recommend Grammar for the Well-Trained Mind. The Purple and the Red student workbooks and answer keys are available now, and the Blue student workbook and answer key will be available in 2019, with a fourth color to complete the series in the coming year. If you are interested in receiving product updates, please sign up for our mailing list/newsletter on the home page of our website.
The same Core Instructor Book is used with each "color" of the student packs. You can start with any color and use them in any order. The workbooks do not increase in intensity (they are not "levels") but solidify the student's grammar proficiency with (eventually) 4 cycles of instruction. The practice content differs in each "color" to keep the student's interest.
You can find more information in this video from the author: https://welltrainedmind.com/c/language-arts/grammar/intermediate-grammar/
This PDF contains the first 6 weeks of the Instructor Text, the Purple Student workbook, the Purple Answer Key, and the Grammar Guidebook. If the link doesn’t work, please copy/paste it into your web browser.