It's a good question. What books do you use? The real question is, how do you mean use?
For instance, several players in my campaign have characters that originate outside of the core rules in terms of feats, spells, magic items and in some cases, alternative core classes. Are those books where those items came from 'In use'?
Otherwise, as a GM, it's mainly monster books. In my Forgotten Realms campaign, I've been using a lot of Dragon Magazine material to help supplement my game. For instance, the Cult of the Ebon Maw has been around since the party was 1st level and they've fought demons from various sourcebooks that were servants or slaves to the Ebon Maw.
Another example is templates. I have a lot of cultist who've become Ebon Servitors (or ebony, I get it mixed up) from the Deluxe book of Templates. Do they count as in use?