Alcohol was never a big issue with Derek, the fact that it had never really had much of an effect on him, that he wasn't really able to use it for escape at any point in his life, meant that he wasn't particularly looking to turn to it now. Sure, he could get drunk now, but it was less of a draw when it could also mean losing his exceptionally tight control.
"Because I thought the library would be the best place to go for a drink." It wasn't like Derek honestly expected anything of the sort, so he wasn't really bothered if Sam had anything around or not. "We wouldn't want accidental spills to threaten the books."
Picking one up, Derek took a seat at the table, stretching his legs out and enjoying the relative quiet too. "What're you looking for anyway?" Unless this was just 'read everything for the sake of it' kind of things.