Do the League books have to be read in order?

Absolutely not. I write all the books as stand-alones and with the assumption that every reader will be picking it up for the first time. I also respect the fact that my worlds are very expansive and complicated and at times a refresher course is helpful even for the most dedicated fan. Especially given that the League books are entering their thirtieth year of publication ;) Lot to remember for thirty years.