I know you said one - but you get two. LOL!
If you are really interested in Paganism, especially Northern Paganism, you should read these books which have a lot of info on the whys.
THE WHITE GODDESS by Robert Graves. This covers many Pagan religions, and where a lot of their ceremonies, traditions, etc., come from. For instance it explains different Gods and Goddesses, Runes, The Tree Alphabet, and how such were used in Bardic Verse and lore.
For instance he tells us the Beth-Luis-Nion tree Alphabet was used in Druidism for divination. He says Nion is the Ash. In Ireland the Tree of Tortu, The Tree of Dathi, The Branching Tree of Usnech. It is also the Great Ash Yygdrasill, sacred to Woden/Oden. The Norns of Scandinavian legend, dispensed justice under the Ash.
He discusses
the War of the Trees, and the magic associated with each. He even points out a tree section in the Bible.
Jdg 9:7 And when they told
it to Jotham, he went and stood in the top of mount Gerizim, and lifted up his voice, and cried, and said unto them, Hearken unto me, ye men of Shechem, that God may hearken unto you.
Jdg 9:8
The trees went forth on a time to anoint a king over them; and they said unto
the olive tree, Reign thou over us.
Jdg 9:9 But
the olive tree said unto them, Should I leave my fatness, wherewith by me they honour God and man, and go to be promoted over the trees?
Jdg 9:10 And the trees said to
the fig tree, Come thou,
and reign over us.
Jdg 9:11
But the fig tree said unto them, Should I forsake my sweetness, and my good fruit, and go to be promoted over the trees?
Jdg 9:12 Then said the trees
unto the vine, Come thou,
and reign over us.
Jdg 9:13
And the vine said unto them, Should I leave my wine, which cheereth God and man, and go to be promoted over the trees?
Jdg 9:14 Then said all the trees unto
the bramble, Come thou,
and reign over us.
Jdg 9:15
And the bramble said unto the trees, If in truth ye anoint me king over you,
then come
and put your trust in my shadow:
and if not, let fire come out of the bramble, and devour the cedars of Lebanon.
The other is
THE GOLDEN BOUGH by James G. Frazer - long, dry, and repetitive, but again, full of information.
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