PBooks is currently the copyright of Savonix Corporation, as I, Albert Lash, have written the program in its entirety. I haven't yet registered the copyright with the Copyright Office but plan to as the code base settles down.
Since PBooks is protected by US law from being copied, how is it able to be copied and used as an open source project? That is through the wonderful capability of licensing. For more information, read the PBooks License page!