The book "Celtic Magic" by D.J. Conway is a horribly inaccurate book full of misinformation. I would personally never recommend it to anyone seeking to learn about the Celts.
The association of trees with birthdates is a modern one and there is no evidence that the Celts used this at all.
The problem with trying to learn about what the Celts did spiritually and magically is that they left no written records. The vast majority of what we know about them was passed down to us by their enemies.What you find in most books about the Celts in books in the popular press is drawn heavily from modern practices such as Wicca and would likely seem completely foreign to an ancient Celt.
Some books on the Celts that would be useful if you want to approach it from a scolarly sound basis would include:
"The Pagan Celts" by Anne Ross
"Pagan Religions of the Ancient British Isles" by Ronald Hutton
"Blood and Mistletoe" by Ronald Hutton
"Dying for the Gods" by Miranda Green
"The Druids" by Peter Berresford Ellis
And two reasonably good books on Celtic practices that I would suggest are
"The Apple Branch: A Path to Celtic Ritual" by Alexei Kondratiev
"Glamoury; Mysteries of the Celtic Green World" by Steven Blamires
Oh, and also avoid the "21 Lessons of Merlyn" as well. Horrible book that claims that all Druids were vegetarians and that a female Druid was called a Dryad when Dryads were Greek tree-spirits. Blech!
I never read that in the 21 Lessons. Perhaps you're thinking of a different book?
As to DJ Conway's Celtic Magic, the mythology found in the book is sound, as I've found found the same information just about everywhere, and that's all I keep that book for. Perhaps you found issue with it, and that's fine, but that doesn't make my advice bad.
But why buy a book if it only has one good part? There are plenty of good books about Celtic mythology out there, and you don't have to deal with the fluff.
Beyond that, you have a good point. All I'm trying to say is that a book isn't all bad just because it has some fluff. And it's good for ME because I incorporate Eastern Philosophies into my own path, which is primarily Celtic to begin with.
I agree with Lark, the biggest problem with Celtic history is that its has been lost. It's fun to read a number of books, but I have not found one that hasn't either been "Christianfied" (Christian doctrine added to make it seem as though the Celts were waiting to be conquered by Christianity) or just other Pagan stuff thrown together to make something Celtic out of another nation's legends. It's all fun to read, but take it with a grain of salt and if the author starts talking about Zeus or using other Greek/Roman/Norse or other terms know their blowing smoke out their bums.