The White Elephant by Margery Allingham

Opening lines:

Mr. Campion, piloting his companion through the crowded courtyard at Burlington House, became aware of the old lady in the Daimler, partly because her chauffeur almost ran over him and partly because she gave him a stare of such vigorous and personal disapproval that he felt she must either know him very well indeed or have mistaken him for someone else entirely.

The Question Mark - Margery Allingham

Opening lines:

When Miss Chloe Pleyell became engaged to Sir Matthew Pearing, K.C., Mr. Albert Campion crossed her name off his private list entitled ‘Elegant Young Persons Whom I Ought to Take to Lunch’ and wrote it in neatly at the foot of his ‘People I Must Send Christmas Cards to’ folder.