Two enchanting memoirs from V. S. Pritchett, available for the first time in a single volume   A Cab at the Door, originally published in 1968, recalls his childhood in turn-of-the-century and World War I London with the urbane subtlety and wry humor that have marked his other works. For the wild and eccentric Pritchett family, life is a series of cabs waiting at the door to transport them to a succession of ten-bob-a-week lodgings, in their flight from creditors and the financial disasters of their father. A Cab at the Door also captures the texture and color of the working-class side of Edwardian England.   Midnight Oil (which Wilfrid Sheed called a "little Rolls Royce of a book" when it came out in 1972) opens in 1921: Pritchett arrives in Paris to commence with a literary career. Gradually, his creative sensibilities emerge as he travels as a reporter to Ireland, Spain, and America. Midnight Oil provides an intimate and precise record of a writer's discovery of himself and his art. "Pritchett is one of the great pleasure-givers in our language," said Eudora Welty.