Any edition of the Student Workbook for Essentials Units 8-15 that is from the 2nd Edition (2015) or later has the same lesson content and may be used with any 2nd Edition or later Teacher's Guide for these eight units. That includes the the current Essentials Student Workbook 8-15 as well as the "Essentials Student Workbook, Volume 1," from 2015, which contained Units 1-15 in one big book.
However, note that books from the 1st Edition of Essentials (published in 2012) are not compatible with newer versions.
Detailed Edition History: Essentials 8-15 Student Workbook
1st Edition: Essentials Cursive Workbook and Essentials Manuscript Workbook: Published in 2012; out of print, but available digitally. 480 pages. All of Essentials (40 lessons) in one book. Single level of application in each lesson. Provided sample answers in cursive or manuscript to model good handwriting.
Compatible only with the Essentials Teacher's Manual, 1st Edition (2012).
2nd Edition: Essentials Student Workbook, Volume 1. Published in 2015. 320 pages. The first half of the updated and expanded Essentials (now Units 1-15, due to major restructuring of the lessons in the 2nd Edition). Includes three levels of application in each lesson.
Compatible with any 2nd Edition or later Teacher's Guide for Units 8-15, including the "Essentials Teacher's Manual, Volume 1" (2015) and the current Essentials 8-15 Teacher's Guide.
Not compatible with the Essentials Cursive or Manuscript Workbook, 1st Edition (2012).
3rd Edition: Essentials 8-15 Student Workbook. Published in 2018. 205 pages. The second quarter of Essentials (Units 8-15). Essentials is now printed in four volumes for manageability and more flexibility in pacing. Lesson content has not changed from 2nd Edition, though minor errors have been corrected.
Compatible with any 2nd Edition or later Teacher's Guide for Units 8-15, including the current Essentials 8-15 Teacher's Guide and the older "Essentials Teacher's Guide, Volume 1" (2015).
Not compatible with the Essentials Teacher's Manual, 1st Edition (2012).
Other 3rd Edition Details
In the second printing of the 3rd Edition, we added a section with guidance about how the three levels work as you move from 1-7 to 8-15. This new content increased the length of the introduction by two pages. All lesson contents were shifted back by two pages as a result. The actual lesson contents have not changed except for minor corrections.