miranadobe:
Oh man, KISMET -- When I read your title my first thot was "Oh man, she's NOT in vogue anymore but there is not a better book on adoption than Carole Lea Benjamin's Second Hand Dog." And Mirandobe puts it out first.
It's got cartoons all the way thru it -- but she makes some weighty points in her cartoons. Things like remember that certain words or behaviors may have been TAUGHT, so so that response YOU are expecting might simply not be what this dog was taught to do. And maybe it wasn't taught WELL -- but you need to be patient in teaching the dog what you expect of it.
The one devil's advocate thing I'm going to mention (and remember that I am a bit disabled myself so I DO think of this) - I don't know you well enough to know how old your mama might be but is it going to be "do-able" to adequately deal with a dog of a rottie's size should it need veterinary care?
Specifically I mean -- when a dog that size gets sick, it may not be able to get into a car/truck under its own power. Is your Mom in a situation where either a vet can come TO her, or is she able-bodied enough (and for the coming 10 or more years) to get a dog of that size TO a vet?
I don't dislike big dogs -- quite the opposite. But I know with my own physical limitations, I've had to face the reality that future dogs for us will have to be under 30 pounds .. and probably under 20, simply so *I* can pick them up if the need arises.
Rushing on to another book suggestion;
The "ASPCA Complete Guide to Dogs" is an awesome tool
http://www.amazon.com/ASPCA-Complete-Guide-Dogs-Aspc/dp/0811819043/ref=sr_1_2?ie=UTF8&qid=1255899259&sr=1-2-fkmr1
It has some real common sense stuff with regard to what you might find at a shelter/animal control -- if a dog looks like ==> it might be mix of this and that breeds. It has a more than decent thumb-nail description of breeds (by AKC group) that is pretty realistic as far as pointing out this dog sheds, that dog needs a more experienced owner, and this breed is prone to patellar problems, etc. It's not a condemnation -- just a pretty brief single-page per breed synopsis that might be helpful?