:iagree:
SL is actually renaming their readers so they reflect grade levels instead of being tied to the cores by number.
What was previously the readers for Core K through Core 2 Advanced are now going to be Grade K Readers, Grade 1, Grade 2, Grade 3, and Grade 4-5.
You can read about that change here:
http://www.sonlight.com/whats-new.html
These levels aren't tied to specific cores or historical themes, they are tied to reading fluency levels.
SL's readers are really a great progression to build fluency with REAL books in the early grades.
HTH!