As the other folks said, there is no one book. Shep Paine's Modeling Tanks and Military Vehicles has some good pointers, but mostly to specific projects.
For, scratchbuildingh, first, be sure the model you want to do do is not available in the scale you want and not coming out soon. NOthin's more frustrating than going through the major effort of a scrathc project then finding the kit on the shelf of the LHS.
Next, go to the design table and lay out your plans. Can you work with sheet stock or will you need to carve a master and cast/vacuform the parts you need? WIl you be turning metal? Do you have the right toolds for the job?
Learn how to scale from line drawings and photographs. Scratchbuilding is really like engineering without the team.
I'd strongly suggest doing some detailing froms scratch before moving to a scratch conversion or full scratch project.
Some basic tools beyond regular modeling stuff may include T squares, miter boxes, razor saws, lathes, straight edges, tube benders.