John I was thinking about this today and then saw your question. I've started reading from book one, and there is a progression. You can start from any book you wish, but beware that there are references to earlier books with a bit of emphasis on the one directly previous to. It's a chain, you can start in the middle but you'd be missing out on the beginning. One thing to note though. The last ones in the series are very good whereas some of the first ones can be a little hit and miss.
Jack Hickey It's an ongoing story. It would be best to start at the beginning for character development.
Kevin Each book has a self-contained story, that explains the characters involved but there are continuing story lines throughout the books. So yes you can start anywhere, but you may want to start at the beginning...
Ajk84 I would say to start at book 1. I later on had to go back to read book 3 because I read book 4 first before it and missed a lot of things that were important. I would stick to the order of the books.