How about a forest biology textbook? I went to a forestry school for
my MS, though my program was more on the biology/genetics side than the
forestry side. Any more specific questions about genetics of trees?
Once you're really looking at DNA, it doesn't matter whether you're
looking at DNA from a human, a tree, or a bacterium - it all works the
same way, and a lot of the sequences are exactly the same, even in
these widely divergent groups. If you mean tree genetics as in
phylogeny and genetic relationships, I think evolutionary history of
trees (or just about any other group of organisms) is pretty
interesting, and I'd recommend a textbook for that too. Or just look
at the evolution chapter in just about any college-level botany
textbook. As far as the sap, smells, chemicals, etc., you'd probably
want to look for something about forest biochemistry or tree
physiology, but those are farther from my area of expertise.
Hope this helps a little,