Welcome from me too.
You may find that you don't need a Guest Assistance Card (GAC) if it's not very busy. We had one in March because DD who uses a wheelchair and has trouble with waiting in line. Even then when it was busy, we only needed to use it once or twice a day.
For most rides/attractions at WDW, wheelchair and
ecv users do wait in line with everyone else (mainstream access). For a few rides, wheelchair or ecv users need to board at the exit and are pulled out of line just before everyone else gets to the boarding area. For the rides where an ecv is not able to stay in the line, the CM at the entrance should trade the ecv for a wheelchair without you needing anything. The problem I can see is that there may not be 2 wheelchairs available to switch to for those rides.
The park maps are coded to tell which rides can accomidate someone staying in a wheelchair or ecv. A little wheelchair icon shows you can stay in the wheelchair or ecv for the whole ride. There is an icon of someone switching from an ecv to a wheelchair for the rides where that is needed and an icon of someone getting out of the wheelchair and standing up when you must actually board the ride car.
Here's a link to the DIS page about touring with mobility disabilities.
There is a
Guidebook for Guests with Disabilities covering each park available from Guest Services at WDW resorts or any of the parks. It's not really helpful about boarding, but does give a little more info about how rough the ride is and also tells how long each ride/attraction. That might be helpful information for your friends, but other than that, it is not real helpful. Other than those, there are no books about touring WDW with special challenges.
If you think of any specific questions, come back and ask.