Wow. Simply, wow. I have spent many years waiting for someone to write a book on this topic.
You can turn over any rock and find a book on Greek mythology, but books about the actual practice of ancient Hellenic religion are not easily found. Finally, Robert Parker fills the gap.
I will warn you that the book seems to have been written primarily for Classics scholars. If you know little or no ancient Greek history and mythology, Parker will not slow down to explain these topics to you. Similarly, it will really help if you can read a little Greek.
But if you can keep up with it, this book is incredibly rewarding.