I have only read book one of the Keeper books, The Water Keeper, so don't know how well the other books are as stand-alone. There is a lot of back story on who the main character is, why he is/what his motivations are, and how does he have the money to do what he does.
I know with the Joe Picket books, one of the early books was important to understanding the overall story arc and although I tried to skip a book, I gave up and waited until I could get the one I was missing.
Other books/characters I'm able to pick up mid-series (like the Reacher books by Lee Child) but some authors/series I just need to go through in order.