While not all of these books have contributed directly to the material presented here, each are books I have read and found interesting and enlightening... some more than others, perhaps... and I recommend each as well worth looking into.

Religion

The Portal: Teachings and Works of Celtic Witchcraft, by Ian Corrigan

Wicca: A Guide For the Solitary Practitioner, by Scott Cunningham

Living Wicca: A Further Guide for the Solitary Practitioner, by Scott Cunningham

Elements of the Druid Tradition, by Philip Car-Gomm

The Druid Way, by Philip Car-Gomm

Druid Craft: The Magic of Wicca & Druidry, by Philip Car-Gomm

Wicca for Men : A Handbook for Male Pagans Seeking a Spiritual Path, by A. J. Drew

Magical Rites from the Crystal Well, by Ed Fitch

The Rites of Odin, by Ed Fitch

Psychic Self-Defense, by Dion Fortune

The Encyclopedia of Witches and Witchcraft, by Rosemary Ellen Guiley

Celtic Mythology and Religion, by Alexander MacBain

Druid Magic, by Maya Magee Sutton, Ph.D. & Nicholas R. Mann

The Western Way: A Practical Guide to the Western Mystery Tradition, by Caitlín & John Matthews

Merlin: Priest of Nature, by Jean Markale

Women of the Celts, by Jean Markale

Celtic Women's Spirituality,  by Edain McCoy

Out of the Shadows: Myths and Truths of Modern Wicca, by Lilith McLelland

The Book of Druidry: History Sites & Wisdom, by Ross Nichols

Elemental Power: Celtic Faerie Craft & Druid Magic, by Amber Wolf

To Ride a Silver Broomstick, By Silver Ravenwolf

To Stir a Magic Cauldron, by Silver Ravenwolf

Celtic Heritage: Ancient Traditions in Ireland and Wales, by Alwyn Rees and Brinley Rees

Pagan Celtic Britain, by Anne Ross

Exploring Wicca: Beliefs, Rites, and Rituals of the Wiccan Religion, by Lady Sabrina

Reclaiming the Power, by Lady Sabrina

Earthlight: the Ancient Path to Transformation rediscovering the Wisdom of Celtic & Faery Lore, by R.J. Stewart (*not* the writer from Xena)

The Power Within the Land: The roots of Celtic & Underworld Traditions Awakening the Sleepers & Regenerating the Earth, by R.J. Stewart

The Underworld Initiation: A Journey Towards Psychic Transformation, by R. J. Stewart

An ABC of Witchcraft, Past and Present, by Doreen Valiente

The Rebirth of Witchcraft, by Doreen Valiente

Magic

Real Magic, by Isaac Bonewits

Celtic Tree Mysteries: Secrets of the Ogham, by Steve Blamires

Glamoury: Magic of the Celtic Green World, by Steve Blamires

Esoteric Rune Magic, by D. Jason Cooper

The Complete Book of Incense, Oils and Brews, by Scott Cunningham

Cunningham's Encyclopedia of Magical Herbs, by Scott Cunningham

Cunningham's Encyclopedia of Crystal, Gem and Metal Magic, by Scott Cunningham

Practical Magic : A Book of Transformations, Spells and Mind Magic, by Marian Green

Astral Projection for Beginners, by Edain McCoy

Spellcraft: A Primer for the Young Magician, by Lilith McLelland

The Salem Witches' Book of Love Spells: Ancient Spells From Modern Witches, by Lilith McLelland

The Complete Illustrated Guide to the Runes, by Nigel Pennick 

Secrets of the Runes, by Nigel Pennick

A Practical Guide to the Runes: Their Uses in Divination and Magick, by Lisa Peschel

Rune Lore,  by Edred Thorson

Futhark: A Handbook of Rune Magic, by Edred Thorson

Natural Magic, by Doreen Valiente