Reddit mentions of The Circle Within: Creating a Wiccan Spiritual Tradition

It's the Winter Solstice, so that's probably why your searching isn't turning up anything. You are so thoughtful to do this for her! What to get her obviously depends on her personality and interests, so if you could tell us a little bit more about her in general that would probably help.

Not knowing what kind of things she already has does make it difficult to recommend things, but I'm going to link you some books that are of general interest but not the kind of thing that most folks already have in a basic library, if that makes sense.

The Return of the Light and In the Light of the Moon are two charming little books full of stories from around the world about the Winter Solstice and the full moon, respectively.

The Charge of the Goddess - The Poetry of Doreen Valiente is a small volume of poetry by one of the most influential women in Wicca.

The Circle Within is a thoughtful, down-to-earth guide for better incorporating Wicca into one's daily life. Very helpful.

The Earth Path is perfect for the ecologically-minded. It shows witches how to really get out in nature and experience it.

A Book of Pagan Prayer is a plain little book packed with a huge variety of prayers and information on creating them.

Theological I would suggest the pagan book of living and dying or the circle within.

History wise you can't really go wrong with any of the other suggestions here.