Although after finding a clear picture on Wikipedia I don't think they look that weird. But then again I grew up thinking that Platypi (Platypuses? Platypodes?) were normal :-D
I think the author really over-does it, though- the inclusion of "bear" is a long stretch; it really doesn't resemble a bear more than other similarly sized rodent species, and north america has had no shortage of gigantic ancestors of modern mammals. Having a bear-sized ancestor is nothing special either (though a bear sized beaver would indeed be a sight to behold!)
Even the references to other animals is a bit much- really, if you took away the funky tail, and remembered that lots of mammals have more webbing between their digits than humans do (otters, even ferrets!) and useful grasping fingers (watching raccoons eat is a delight)... well, they look an awful lot like a rabbit without the long ears.
As for their colony building, there are plenty of other animals that build colonies, though beavers have a somewhat uniquely semi-aquatic behavioral adaptation in the way they build dams.
They really are fascinating creatures, but also terribly destructive if they decide your bushes or trees look interesting. I've got metal wire mesh wrapped around several trees to protect them from the buggers.