I have a celtic Ogham set and have read a few books on it, and the relevant material in the White Goddess (Robert Graves).
I find it is a fantastic oracle, I used to use tarot mainly and runes second, but Ogham has way taken over as my second most used form of divination.
What I like about Ogham, in similar with runes, is the straightfoward answers. There's no beating around the bush, or twisting ther interpretations, it's in your face!
At present I am using a set all made of one wood, but with the characters carved into the fews, but I am currently collecting wood to make new fews, with each few made of its appropriate wood. I personally feel it is fine to take wood from the tree as long as you ask permission and leave something in return. I personally leave a handful of tea leaves or such like in the ground, which will break down to become food for the tree, rather than littering it with crystals, coins and ribbons. An alternative is to send it some healing energy. May I ask if any of you have a favourite tree ogham? Mine is probably Birch & Hawthorn. I love Birch because of its association with Sheel Na Gig (the main Goddess I work with) and Hawthorn because of its
gnarled appearance and association with Beltane (one of my favourite times of the year!).