PCI SIG
The fundamental purpose of the PCI-SIG is to deliver a stable, straightforward and compatible standard for PCI devices. PCI-SIG members may download the specifications for free; non-members are invited to purchase the specifications. For more information, please visit the Ordering information page.
http://www.pcisig.com/specifications
The Specifications are grouped into three areas. In each area, there are specs, white papers, and related presentations.