The textbook is correct, at constant EAS
The thing that I find is often not grasped is this: all the above is of course correct for best range cruise, or indeed for cruise at any constant EAS as Brian has said. However, the way light twins are usually operated is very much not constant EAS and very much faster than best range.
If you're flying at a high cruise power, the range/altitude graphs change markedly with altitude which can be confusing for someone trained using AFNA as a standard text. Not that AFNA is wrong, just insufficient by itself for a full appreciation. It's important to be able to appreciate why flight manual graphs look so different.
Cheers,
O8