pp. 521-522 are printed in the wrong order.
When The Day After Judgment was originally published in a single volume, it had a four page Prologue which began:
"The events leading up to the disaster were as follows:..."
-James Blish, The Day After Judgment (New York, 1971), p. 9 -
- and which then summarized the plot of Black Easter. This Prologue, presumably regarded as no longer necessary, was omitted when The Day After Judgment was published in a single volume with Black Easter and then here in the one-volume After Such Knowledge. However, I think that this impersonal summary adds to the work and should be retained.
For a discussion of ASK, Vol I, Doctor Mirabilis, on the Poul Anderson Appreciation blog, see here.