I have the SC Grail and
Manhattan GMAT's guide books. I personally don't think the Subject/Verb Agreement and Modifier chapters of the SC Grail are so important that you need to shell out another $25, especially if you are a native speaker.
Those types of problems center really on understanding the meaning of a sentence and how the modifiers function. For a free resource, I would suggest the GMAT Club Grammar book. I would also suggest the Thursdays with Ron (which can be tediously long, but helpful... I FF through some stuff to keep it moving) videos for helpful review.
I'm finding that I was too cavalier with the SC section in my studies since I tested relatively high on the diagnostics. GMAT sentences are a pain so you really need to be focused on deconstructing/meaning.