While I wouldn’t exactly call myself an expert, I have been a Pagan for about 30 years. These are a few resources I would endorse for Pagans just starting out, especially those on a Celtic and/or Norse path.
People of the Earth: The New Pagans Speak Out by Ellen Evert Hopman and Lawrence Bond
Including interviews across the spectrum, this book is a great way to compare the various Pagan paths (even though now it has become a little outdated, as new paths have arisen over the years). Many of the interviews are with founders of traditions, making this compilation of great historical significance as well. A similarly good source is Drawing Down the Moon by Margot Adler.
Earth Power: Techniques of Natural Magic by Scott Cunningham
This book has stuck with me through the years. These simple techniques in natural magic are of benefit to anyone of any Pagan path. The earliest editions of this book have beautiful art nouveau illustrations.
Celtic Rituals by Alexei Kondratiev
I recommend this not for the ritual style, but for the deep insights and ritual themes Alexei had for the holy days.
The Celtic Devotional: Daily Prayers and Blessings by Caitlin Matthews
Recommended especially for soft polytheists or duotheists, this little book is a treasure trove of beautiful of prayers and blessings.
Kindling the Celtic Spirit by Mara Freeman
A nice source and light read for customs and themes of various holidays throughout the year.
Myth, Legend, and Romance: An Encyclopaedia of Irish Folk Tradition by Daithi O Hogain
A good solid reference.
The Poetic Edda by Carolyne Larrington
A clear and readable translation of the Elder Edda.
The Children of Odin by Padraic Colum
Yes, this is a children’s mythology book, but a great read and easy introduction to Norse mythology for adults as well. It’s a good place to start if you’re not ready to tackle the Eddas.
The Seed of Yggdrasill- Deciphering the Hidden Messages in Old Norse Myths by Maria Christina Kvilhaug
Maria has some really mind-blowing amazing insights into Norse mythology. If you can’t find her book at a reasonable price, check out her youtube channel: Lady of the Labyrinth.
Sea Sky Soil: An Introduction to Waincraft by Nicanthiel Hrafnhild
If you lean more toward soft/squishy polytheism, and want to combine Norse and Celtic pantheons, this book will provide a lot of ideas and inspiration for it. However, does not include any guidelines on forming Waincraft ritual.
The Witch’s Voice (Witchvox)
A great source for descriptions of Pagan paths, articles, and finding local Pagan events and groups (not just for Witches/Wiccans).
Ár nDraíocht Féin: A Druid Fellowship
A great organization and a great resource for rituals, chants, articles, and Indo-European Paganism.
The CR FAQ
A good place to start if interested in Celtic Reconstructionist Paganism.
The organization is no longer active, but the website is a great source of articles and information.
Their name means “sacred kinship”. A great source for Gaelic polytheism.
Temple of Our Heathen Gods
So much information and resources. So. Much.
Ásatrú basics, very informative for beginners.
A wonderful online source for Norse poetry, invocations, and especially- song.
Waincraft – Following the Call of the Land
The main online source for this fairly new tradition.
The Compleat Waincraft
Waincraft on tumblr.