Hello China!
As a birdy beginner your Mum doesn't want to be overwhelmed with stuff that wouldn't interest her - at the moment, that is - plenty of time for that later. THE book that would be right is The Mitchell Beazley Pocket Guide to Garden Birds by Dominic Couzens and Mike Langman. it really is a terrific book with plenty of information about garden species.
http://tinyurl.com/lwesn4
The other one which your Mum might like is the RSPB book just called Birds. It's published by DK, so the colour is superb. Either of these would be suitable.