Doug, I've tried to emphasize before that creating your own spam filters is NOT the best way to fight spam in GMail. I'm not surprised that you're seeing this behavior. I'm sure that it's one of your filters that's causing the problem.

I think that whether or not GMail scans for those terms in attachments will depend on how you've created the filter. GMail is capable of searching attachments when you do simple mail searches, for example, so it's certainly capable of it, though it probably also depends on what file format those ebooks are.

I strongly recommend that you remove all those filters and let GMail do its thing. It's going to scan for spam anyway, so just spend a couple weeks marking spam using the built in tool and I promise that after a while you won't see any spam. I haven't seen any in years. The only thing I get is the occasional well-worded phishing scam, but it's tough for GMail to find those unless enough people mark it as phishing.