Can we just install prince normally and put the prince-for-books executable next to it, and run either one, or do they each need a distinct whole installation?

They have different data files so they need distinct installations.

But for most things using prince-for-books should work the same as prince, right?

Yes, although the current release of Prince for Books does not include all of the features found in Prince 16, we are working on this.

Is there a list of missing features?

It should be compatible with Prince 15.3.