The first is from Consul Books and is simply titled The Avengers. It was written by Douglas Enefer and was published in 1963.
This is the only original novel based on the partnership of Cathy Gale and John Steed.
This next one is titled Dead Duck and is credited to Patrick Macnee, although inside, author Peter Leslie also gets a namecheck
This book published by Hodder and Stoughton in 1966, features the pairing of Emma Peel and John Steed.
A second book in this series was released the previous year prior to Dead Duck called Deadline.
My example is a reprint by Titan Books which was published in 1994.
In 1967 Panther Books released four original novels all written by John Garforth, again featuring the partnership between Emma Peel and John Steed.
These four books were reprinted by Berkley Medallion in 1968 for the American market with a further five original titles not available in the UK. One was another featuring Emma Peel along with four featuring Tara King.
I only have two so far. The Afrit Affair, written by Keith Laumer which features Emma Peel and John Steed.
The Magnetic Man , written by Norman Daniels, which features Tara King and John Steed.