Thanks for all your guesses! The answer to the burning question of "what the heck was that thing" is, it was a local phone book.
Yes, the phone company delivered one of those small local phone books in a paper bag to the office. It got rained on and driven over multiple times. Now it is just a flat mass of whitish, grayish and yellowish pulp with a bit of plastic embedded inside. On dry days, the entire thing can be picked up and moved, which is just what I'm going to do with it. Move it to the dumpster.