This is really going to depend on what sort of character the Wiccan is. That is, are they new at this, an old hand, wide-eyed idealist, young, old, male, female, feminist, earth-centered, Celtophile, etc.?
While there is a legal Church of Wicca, what you’ll find in reality is that most “Wiccans” have their own filters and ideas about the universe.
Scott Cunninghan or Ray Buckland will give you the basics. The rest is details.
A hard-line feminist would probably be drawn to Dianic Wicca and would definitely have read Z. Budapest, particularly The Feminist Book of Lights and Shadows.
A newbie Wiccan in the '80’s or '90’s probably started with Wicca: A Guide for the Solitary Practitioner by Scott Cunningham or To Ride A Silver Broomstick by Silver Ravenwolf (The later considered something of a joke by serious practitioners, but undoubtedly influential, especially among young practitioners.)
Male Wiccans seem drawn to Ray Buckland’s Buckland’s Complete Book of Witchcraft (known to us as “Bucky’s Big Blue Book”)
For Celtic Wicca, beware your source. There is a lot of total shite out there. Like the reviewer at Amazon, my reaction to Celtic Wicca: Ancient Wisdom for the 21st Century by Jane Raeborn was “Look! It’s not crap!” Not exactly a ringing endorsement, but I was highly impressed with this unknown author.
Kerr Cuhulan is a newer author, and is bringing some exciting new ideas to the table around Wicca not being a fuzzy lovey new agey religion. Lots of military folks are excited about his works, including Wiccan Warrior. If your character is going to be using Wicca offensively, you need to read his book.
Christopher Penczak is a guy I generally respect, though I haven’t read this book, it seems like it might be useful to you as well: Outer Temple Of Witchcraft. I did read and liked his book City Magick
While most Wiccans are rural in mindset, regardless of where they live, there are a few authors writing exciting new ideas about urban Wicca and paganism, and how those of us trapped in the city can practice our religion with what we interact with everyday, instead of envisioning some bucolic mountainside. My favorite of these is Urban Primitive. But - full warning - it is unorthodox, and if you were to call it “Wicca”, you’d have some explaining to do.
If you don’t know Raven Grimassi, you’ll never be taken seriously. His strong points are hereditary witchcraft and Italian withcraft. And yes, he’s a guy.
Drawing Down the Moon: Witches, Druids, Goddess-Worshippers, and Other Pagans in America Today by Margot Adler is an anthropologic study of wicca and neopaganism in America. It’s a little dated, but still very good.