The Avengers Novelisations at the Internet Archive
These novelisations have come up here a couple of times. There were a number of novels published based on The Avengers in the sixties. Rather than rely on the remaining yellowing paperbacks, it is my joy to let you know that many of these are available at the Internet Archive. I have worked from this list to compile this list, and there may well be others. I have read some of them but look forward to expanding my reading.
Deadline by Patrick Macnee and Peter Leslie (1965)
https://archive.org/details/TheAvengersDeadline
Dead Duck by Patrick Macnee and Peter Leslie (1966)
https://archive.org/details/TheAvengersDeadDuck
The Floating Game by John Garforth (1967)
https://archive.org/details/avengers1thefloatinggame
The Laugh Was on Lazarus by John Garforth (1967)
https://archive.org/details/avengers2thelaughwasonlazarus
The Passing of Gloria Monday by John Garforth (1967)
https://archive.org/details/avengers3thepassingofgloriamunday
Heil Harris by John Garforth (1967)
https://archive.org/details/avengers4heilharris
House of Cards by Peter Cave (1978)
https://archive.org/details/houseofcards00cave (this is available to borrow rather than download, but you just have to sign up for free and they don't give you any bother)
Too Many Targets by John Peel and Dave Rogers (1998)
https://archive.org/details/avengerstoomanyt00peel
They also have Patrick Macnee's autobiography, Blind in One Ear (1989) to borrow: