Touringplans.com is one of the top, as he is also part of the Unofficial Guide. Cost is pretty low, at around $15. There is also the Easy Guide to Your Walt Disney World Visit. It is a book, but does explain everything and offers THE BEST OF while determining your schedule. I bought 2 used that were 1 year old, so slightly out of date, but still very usable. I lend them to friends that are going to help them learn about their trip. Check used books online. I see the 2018 version on thriftbooks for $4.79, 2019 for $5.79. Find at least $10 worth of books and it ships for free.
While early SEP is a good time to visit, you will also find that it is REALLY hot. Food & Wine will be running, and that is fun. While it will seem much lighter just walking around, the waits for rides will be lower, but not as low as 10-15 years ago.