Explanation of classifications??

[iamppdoc]

I have kept orchids for many years and have finally decided I want to know more about their classification and interrelationships. I am looking for a book that will help, in general terms, with this understanding.

1. What, exactly, is an orchid? What makes it an orchid?
2. Why does this or that orchid fit into a certain tribe or sub- tribe?
3. How do different genera, or tribes relate?
4. It would also be nice to read, again in general, about the origin of genera and their culture.
I know you can find a lot of this on line but it is scattered all over the place and hard to pull together.

Any thoughts?
Thanks, Ed