I'm looking for a book to learn more about growing and transforming your own wood for garden structures. Like how to bend (willow?) branches into loops for polytunnels, or which trees to grow for sturdy poles and how to use them for longevity, or which branches are longer lived as sides for raised beds, or how to use twigs to make a supporting fence for plants to grow on, etc.
The best place I've found for that sort of information is Mother Earth News.
The best time to bend any wood (sawn or raw cut branches) is when it is green, that is when it's in the most pliable state.
After that, it is time to either soak till saturated with water or steamed so that the fibers will relax and bend without separating or shattering
Wood is made of cellulose and that is made of Lignin, when dry the cell structures are very rigid, even when wet they are pretty rigid, that's what holds trees and plants in an upright position.
When bending it is important to take time, speed is the enemy when bending wood. Twine or some other binding material is how you hold things in the shape you want.
If you want to build something like a "bent wood chair" then you need a drill and either nails (brads) or screws to put the pieces together and hold them in place.
Look for books like the Fox Fire series in your bookstores, you might do a search for those or "mountain furniture" or Bent stick furniture and structures.
Willow, white oak, hickory and poplar are the traditional woods for green wood furniture building. Others that work pretty well are Iron wood, Catalpa and Osage Orange.
If you find a "stick" you like the look of, try bending it while it is still living, if it bends something like you want in the finished product, then it will work for your needs most likely.