Note:

• Revised as Dry Spell, 1967?
• Four Square Books published The Drought and Dry Spell with identical page count 157 --from WorldCat library records.
• From the series bibliography at SFE3 (quote): • The Drought: A New Dr Palfrey Story (London: Hodder and Stoughton, 1959) [Dr Palfrey: hb/]   • Dry Spell (London: New English Library, 1967) [rev vt of the above: pb/] Here "rev vt" is revised, variant title. Compound listings at SFE3 show first printings only revised works and variant titles (no 1st US ed., 1st paperback, etc).
• The Drought (#21) was followed three years later, a long wait in this series, by The Terror: The Return of Dr. Palfrey (#22), which later became the first of more than 20 volumes published by Walker & Co. for the US market. One US review of the latter (below) explains that The Drought had concluded the story arc, apparently, with the death of Dr. Palfrey.   So a revision of The Drought may be no surprise--but Sherlock Holmes returned without any published rewrite.

Synopsis: Next in the series (quote): The Terror ... brings back Dr. "Sap" Palfrey who presumably perished in The Drought as did his wife Drusilla and most of mankind when the waters of the world dried up. Interest in Palfrey apparently never played out and he's back, a la Sherlock Holmes and just as busy. --review in "Crime on My Hands" by Vivian Mort, Chicago Tribune 1966-05-22 pN4