Book recommendations for worriers?

[Outcomesthebunnyofdeath]

My mum has always been a dreadful worrier. My nan (her mum) died recently and obviously she is grieving which seems to have exacerbated things. She feels particularly bad at night, thinking things over and going over all kinds of stuff in her head. Anything from feeling guilt that she wasn't able to look after nan at home (she did, for over a year but nan had dementia and mum was on her knees. She thrived once in her care home and it was completely the best thing to do but mum still feels the guilt.) To wanting to go on holiday and worrying about terrorist attacks, who will get my niece to ballet exams if she goes etc etc.

I'm not sure at which point worrying becomes anxiety - or even if they are much the same thing. I wanted to get a book for her and have ordered Dale Carnegie's "How to stop worrying and start living" but wondered if anyone has any other self help recommendations that may help.

Thanks in advance :)