If you want good introductions to Greek religion, I recommend Walter Burkert's Greek Religion and Krindt's Rethinking Greek Religion.

That said, none of these books will help you practice "classical paganism." First of all, that's because there is no such thing as classical paganism. "Paganism" is a pejorative term that was used by Christians to refer to polytheistic religions of the ancient world. And there were a variety of polytheistic religions and practices that changed greatly over time. Unlike Judeo-Christian religions, these were not text-based religions: there was no Bible, Torah or Koran to guide the worshipper, and generally no dogmas that were contested.

Second of all, even if you wanted to, say, practice Athenian religion of the fifth century B.C., you wouldn't be able to, even if you decided to worship gods called Zeus, Hera, Athena, etc... Much of "ancient Greek religion" was based around your city; the festivals celebrated, the rituals, the local variants of individual gods. For example, as an Athenian you wouldn't just worship Athena, you would worship Athena Polias (Athena of the City), and in a different city you'd worship a different version of Athena. Though there were private forms of worship and mystery cults, and though you could certainly participate in other cities' rituals, it can't be understated how much religious worship was tied to the city and communal ritual. And that framework no longer exists. Not to mention that animal sacrifice was a key part of such worship, which might get you into some trouble in these days... Though later Neoplatonist authors, such as Porphyry, began to downplay the necessity of animal sacrifice in ritual. Again, a lot changed over time, and there were lots of different practices.

I hope you enjoy reading about Greek religion anyway. I know very little about modern pagan movements, but I do know they have very little in common with anything done in the ancient world. That said, if they make you and other people happy and you believe in them, then you should do what makes you happy. Just know that learning about ancient religion may not have as much of an effect on what you can practice now as you may like.