Lets make a few assumptions first
You will plane the boards yourself
The bookcase will be 7ft tall, 3ft 6" wide 12" deep carcase with 6No 10" deep shelves
All boards 1" nominal thickness
Kiln dried american white oak (Square edge)
This would require the following
Sides 2No @ 84" x 12" nominal
Top 1No @ 42"x 12"
Bottom 1No @ 42" x 12"
Shelves 6No @ 40" x 10"
Unless you want to pay a high premium and have a wide surface planner it will be best to glue up all the panels from two boards. Also allow extra length for planner snipe say 6" per item.
I would ask for 6No 8ft boards 6" wide x 1" thick (sides, top & bot) and 6No 8ft boards 5" wide x 1" thick(shelves) plus any timber for skirting/base and cornice, not forgetting a sheet of 6mm oak veneered MDF for the back.
Expect to be supplied with boards that may be bigger than you ask for as the supplier will give you material to get what you want out of with the minimum of waste ie some may be 9ft long while others may be an inch or so wider(you will be charged for the total volume of oak supplied). If it were English oak I would allow more for waste and waney edges.
This is also explained somewhere on the site that I posted the link for.
http://www.slhardwoods.com/looseboards.aspHope this helps, Jason