The Man From U.N.C.L.E. No. 6 - The Dagger Affair by David McDaniel (Four Square, 1965)

 

"Tell us all about Dagger!"

This was the order threw at Napoleon Solo and Illya Kuryakin.  And from each U.N.C.L.E.  agent came the same answer: 'We know absolutely nothing of DAGGER.' 

'You appear to be telling the truth,' said a hidden voice.  'A pity...'

But it was more than merely unfortunate that the U.N.C.L.E. organization had never heard of DAGGER--for the secret behind that name was an insane plot for mass murder of the human race!

Often regarded as the best of the tie-in novels based on the 60s TV series, The Dagger Affair is a fun read.  Good guys must unite with bad guys to defeat a common enemy.  Of course, the bad guys remaining bad guys (be they never so entertaining) the truce comes to an end once the common enemy is defeated.

This is the book, incidentally, where evil organization "THRUSH" gets a fuller form of its name: The Technological Hierarchy for the Removal of Undesirables and the Subjugation of Humanity!