Thank you to those with suggestions about the books. I think the sensible way forward is to ask my mother to gradually sort out the ones she does not think she will give to friends, or relatives and take them to Charity shops set up to cope with valuing books etc, eg Oxfam and Acorns (a local Children's Hospice).
I have already had the challenge of sorting my father's possessions, which seemed to take a very long time, and a lot energy, not to mention the emotions involved. More recently, I had to assist with the clearance of another relative's house. I absolutely will not throw away anything that can be of good use to others, whether or not it has value. The e-bay idea is a good one, but I have only recently got round to buying things on e-bay, so would not know how to begin, nor would I know the value of what I was selling, so in my case, this may not be practical. Presumably the Charities would value the books correctly and get a fair price for them.