I really thought I was going to be without a good Language Arts spine for the fall until the folks on the Mater Amabilis Teacher Training forum talked about Emma Serl's Primary Language Lessons and Intermediate Language Lessons, both available in beautiful editions at Hillside Education.

The books were written in early 20th century America.  They contain a wonderful mix of instruction, picture study, narration, dictation, poetry, and composition broken down into many short oral and written lessons, enough to cover 2 to 4 years of instruction depending on how you use it.  The style is much more learning by doing rather than drilling on a concept.  Hillside's version is a beautiful color version, spiral bound, and updated, including a touch of Catholic content.  Straight reprints of both Primary Language Lessons and Intermediate Language Lessons are available through Amazon.

There's much more content compared to Simply Grammar, yet flexible enough to add in other sources and concepts with it--just like a good spine should be!