Spitballing: Hollywood is really just a grown-up version of junior high. The good looking people get all the attention. Those who aren't as aesthetically gifted are the misanthropes who, in an attempt to achieve a modicum of acceptance, will write papers (or in Hollywood, scripts) for the popular folks. Given Hollywood's similarity to junior high, it should come as no surprise "spitballing" is part of its parlance. Here it means throwing out random ideas (often for stories) in hopes that one of them doesn't suck.
Spec: It means a script that no one has paid you to write. You just did it in hopes that some executive somewhere might like it just enough to then rip you off. Let's hope you have a good lawyer.
On the Books: The last time someone in Hollywood read a book, it was ... ok ... maybe never. In fact, no one in this town reads more than a few sentences (log line, script lines, sexually explicit text) before taking a cocaine break. So we'll ever know the etymology of this term. My guess is it probably resulted from a drug-fueled game of Balderdash. Wait. What was I talking about? Oh, right. The term "On the Books" means you've scheduled something for your boss.