Do your books need to be read in any certain order?
Nope! The beauty of a mystery is how the mystery/crime is wrapped up in that book. The only reason they are considered a series is due to the setting/location in the novels, the main sleuth is the same as well as the secondary cast of characters.
If there is a subplot that is threaded throughout the series, there’s a preview to let brings you up to date woven in the story so you aren’t lost.