I was wondering about that while reading the article... Does he really have a complete Amazon book price database on his PDA? If so, how does he keep it up-to-date? I imagine Amazon would object to someone constantly scraping their prices.
the amazon XML api is really nice - it was my primary tool when I was working in that space north of five years ago.
I don't know what this guy did, but what I did (and this would become more useful if I had more clients than just me) was that I cached the data for a certain period of time... so the first time I asked for it within a certain time period it'd actually ask, the second or third time it'd hit a cache.
Now, if I was making software for other people to do this, and I had a bunch of guys like this one using my software to search for books, assuming it went through my db before amazon's, you'd build up a pretty large cache.
Of course, if you could somehow arrange for the whole damn thing to be available offline, that'd be even better, but I don't know how amazon would feel about it. When I was doing it, there were rate-limiting things preventing me; but if you are bigger, there's really no reason for amazon to not give you the data if you are willing to pay for it or what have you; this product is ultimately helpful to amazon.com as a merchant.
