You can download this TI document which is based on IEEE 91-1984 and has a lot of what is in that spec. In order to get the full spec you will need an IEEE membership, but I'm sure if you do a Google search on either IEEE 91-1984 or IEEE 91a-1991 you will find something. I do believe I have seen both of these specs posted in this forum too, so you might do a forum search. These standards are pretty dated, but they have good basic information.
I have heard that the IPC is starting to work on a new specification, but I haven't heard too much. Maybe if there is someone from PCB Matrix (Formally PCB Libraries) on here they can comment on that since they have a good link in to them.