。。
I'm not going to the library. But I'll take you as far as the station.
Literal
Library [topic-は] don't-go [explanation-んだよ]. But, station [until-まで] [if-なら] take-[for-you-てあげる] [よ].
A refusal followed by a partial concession — a very common Japanese discourse shape. 図書館は preserves the topic is elided 'library,' actively excluding it from the following affirmative. までなら then hedges 'as far as the station — under that condition' as a conditional offer. 連れてって is the casual contracted 連れていって. ~てあげる marks the favor as going 'from me to you.' The pairing of a flat refusal with the softening offer is a characteristic face-saving move.