Do I need to read the tie in issues or can I just read the 12 main books of the series?
Read the tie in issues. It helps to know some context.
I’ve read the 12 main books before without the tie ins and they were some things I was confused in. I believe the reading order on this app is inaccurate (unless they did fix it). Google the real reading order.
I just read the twelve issues and I was fine, but if you want to be sure you get everything, the tie-ins are probably a good idea.