Finding good books on modern polytheism can be a difficult task. There are plenty of books detailing ancient worship, but those focused on modern revivalism are often too basic to be of interest to longtime practitioners or full of neopagan loopiness invented whole cloth by the author. I am happy to say that Kharis: Hellenic Polytheism Explored by Sarah Kate Istra Winter is one of the rare exceptions.
Winter's knowledge of Hellenic religious tradition is very impressive. She draws from many scholarly sources (with inline citations) and provides a thorough overview of all major aspects of Hellenic religion, as well as ideas for how they may be adapted for modern practice. She also corrects some common misconceptions about Hellenism and provides insight to often overlooked aspects of Hellenic worship, such as the worship of nymphs and agathos daimons.
Unlike Hellenic Polytheism: Household Worship, Kharis is also a deeply personal work. Winter describes her relationship with the Gods and how worship has strengthened that relationship over time. I don't agree with all of her personal views (for instance, I am much less open to mysticism); however, even those I disagreed with gave me plenty of food for thought, and Winter always clearly delineates which elements are based in tradition and which are her own additions.
One point I agree strongly with Winter on is that modern Hellenism must be a "living religion." In the ancient Greek world, religious practice evolved in response to momentous events or perceived influence of the Gods, and there was a great variety in worship among different cities. Winter encourages practitioners not to just obsess over surviving ancient sources (which are mostly limited to works from Athens) and instead to build a relationship to the Gods unique to the individual or community. As modern polytheists, we shouldn't take the neopagan route of 'anything goes'. Our practice must be rooted in tradition, but we also must not just be play actors carrying out ceremonies with no underlying connection to the Divine.
There are several useful appendices at the end of the book, including a brief glossary of Greek terms, a list of plants, animals, and places sacred to specific Gods, and some internet resources for further reading. Winter also includes a survey of Hellenic polytheists she took in 2007 describing their demographics and approach to worship. While it is dated and the survey only had 166 respondents, I still found it interesting how much variety there is among Hellenists even in a relatively small group.
Kharis has quickly become one of my favourite books on Hellenic polytheism. I devoured it in only two days. Whether you're just starting out or are an experienced practitioner, I highly recommend adding Kharis to your bookshelf.