@donperreault Funnily enough, this was the first book I've read within the series. It was the 'book of the week' from Apple Books a few weeks ago, so it was £3.99 or £4.99 so I picked it up.
I'm not sure if other books go into it more, but it didn't really explore the character himself, other than describing what he looked like and a little about this past (just that he used to be an MP in the Army), so it doesn't feel like I'd missed a whole lot. It felt like a very self-contained story, with a defined beginning and end. It seems there are a lot of Jack Reacher books in this series, so I can only assume they are all designed to be self-contained.