In a few days Mr. Bingley returned Mr. Bennet's visit, and sat about ten minutes with him in his library… (Chapter 3)

They found Mr. Bennet still up. With a book he was regardless of time… (Chapter 3)

Mr. Bennet entertains Bingley in his library rather than a drawing room. I do not suppose that is odd; probably the ladies are in a drawing room which Mr. Bennet wants to avoid anyway. He also preferred to stay home and read rather than attend the assembly ball—perhaps not an odd choice but it does show he would rather be with his books than with people. ;D