Lucius is used to being on the giving, rather than the receiving end of condescension; he's been trying to behave, but Mycroft is inadvertently probing a point he has no way to know is sore. Lucius had simply meant "new to the library" - he was there enough he felt the guess was fair - but the bigger issue is that prior to Baedal (and his previous out of world stops), Lucius had simply never had to dealt with not being given automatic recognition and respect, or at least fear.

He knows he shouldn't care, but he can't quite help it.

"I do hope you'll deign to forgive the unasked-for interruption, then, as you're managing quite well." It's bone dry. "I'll let you read." And resist the urge to do something as petty as squeak the floorboard at random intervals. Because he is an adult.