The Well-Tempered Clavier

"At an early age I found my father's copy of Bach's forty-eight Preludes and Fugues for the well-tuned piano, and studied carefully the arrangements of sharps and flats in the key signatures. My father explained to me how Bach worked his way twice through all the twenty-four major and minor keys. But why is there no prelude in E-flat minor in the second book? My father did not know. Bach just decided when he came to No."