This is a helpful, clear, and (relatively!) simple approach to verbal aspect in Greek. The concepts are relatively challenging, especially for those who haven't come across aspect properly in learning Greek, as is likely the case for users of anything but textbooks published in the last decade or so. But Campbell gives the simplest explanations I've cover across.
It's not a book that argues for the position it takes, despite the fact that some of this is quite contentious; and at times I did want to know the alternative.
What it does do is give excellent, concise descriptions of how different Greek 'tenses' actually work, followed by a bunch of excellent examples with translations, and a few simple exercises.
The concepts here are radical if you haven't come across them (there are no 'past tenses' in Greek; the Perfect is imperfect... And so on) but they do make real sense. More importantly, they give tactical reasons to all the mysterious uses of tense, and apparently random tense changes, that you find when you start reading Greek. After most textbooks, these make little sense. After this little book, they do.
Reading this will keep you from a large variety of misreadings and explain and nuance many New Testament passages. It makes the New Testament clearer and more powerful. So it's hard not to recommend!