A whole bunch of Douglas Preston/Lincoln Child books are on sale for 99p on Amazon UK, including books 0-5 and 13-17 of the Agent Pendergast series.
Extraction (Agent Pendergast Book 0)
https://smile.amazon.co.uk/gp/product/B00NLGBJ1E
Relic (Agent Pendergast Series Book 1)
https://smile.amazon.co.uk/Relic-Age.../dp/B00NLGBJAU
Books 5-7 are also available as an omnibus for £3.99, which is cheaper than either 6 or 7 on their own.
https://smile.amazon.co.uk/Diogenes-.../dp/B00FOW9KSC
Similarly, books 10-12 are available as an ombinus for £3.99:
https://smile.amazon.co.uk/Helen-Tri...dp/B01LW58K40/
I believe that leaves only books 8 and 9 not not sale, they are £4.99 each:
https://smile.amazon.co.uk/Wheel-Dar.../dp/B004JHY8XA
https://smile.amazon.co.uk/Cemetery-.../dp/B004JHY8OO