Mike
I do have the book and am currently cutting the frame.
1) Is this a good book?
yes a very good book from a beginners point of view
2) Can I make a 12x32 garage simply by adding two bents in this plan?
Yes but as the design has continuous plate you will need two 32 foot 8X8s or be able to do a scarf joint
3) Does this have a loft with at least 4' walls on the outside?
Over all floor to top of plate is 10 feet, top of tie beam is 8 feet so No it is more like a 2 foot knee wall
4) Can anyone venture a guess as to the frame cost for this? I'll have it cut at a local sawmill.
An identical shed (12x16)was just posted in an add as $5000 jointed and delivered (not erected)
5) Is cottonwood good for the frame? I was at least going to use it for the sheathing and roof underlayment.
Don't know enough about it to comment
6) Can I build it without a concrete floor?
It is actually designed with a wood floor. You would probably want to add some carring beams the width of the car under the joists. On mine we dug 3 foot cubed holes and filled them with stone and then set 12x12x8 block on that. A layer of 6 mil plastic under the whole building and 4 inches of stone on that. Gets it up off the ground and has good drainage under it, hopefully limiting shifting due to frost.

Hope that helps
Bob