A Caution About WGA
Keep in mind that WGA can only deal with the TV/film industry. It has no jurisdiction over, for example, plays; and it doesn't care. (In fact, it's not allowed to care, but that's a matter of labor and antitrust law.) Some WGA signatory agencies bend over backwards to follow the letter of the rules for dealing within that industry, and {insert graphic description of deviant sexual practices here} the writer/client for anything outside that point. It's not as rare as it should be to see WGA signatories charging substantial up-front fees and higher-than-normal commission rates to its clients when peddling books instead of scripts.
Unfortunately, too, the WGA is extremely slow to react to problems, and the less said about the integrity and speed of its system for allocating credits the better. Sadly, this is about par for the publishing and entertainment industry.