The information in this section is not applicable to all areas. As a best practice, you should read up before doing anything stupid. As usual, this page does no constitute legal advice.
You can still get the book from the feds listing state and national firearm laws, get it and know it, the laws they are always a changing.
You can legally transport an unloaded and dissembled firearm anywhere in the United States ignoring local and state laws if just traveling through from one location where you are allowed to have that weapon to another place which also allows possession of that weapon, as long as it is unloaded in the trunk of your car or if the vehicle has no trunk in a locked container that is not the glove compartment or console according to GORRA-1986.
You may still make private off the books purchases between individuals for privately owned firearms if you are allowed to have those weapons (in most locations).
You are allowed to make any firearm that you can legally own for your own use (in most locations).
The legal definition of a firearm according to the feds is typically the frame or the receiver. There are 80% complete receiver and frame kits that you can order along with the rest of the parts to make a legal firearm 'off the books'.
Any questions, contact a competent firearms law lawyer, or just trust that the constitution will protect you...