A self help book for my DM

[Charliesangel2009]

My DF left my DM 25 years ago. It was pretty traumatic, not least because he left her for a woman 20 years younger. But their marriage had been a sham tbh for a while, with both of them having affairs.
My DM married on the rebound a couple of years later but that marriage broke down, unsurprisingly, a couple of years ago.
And to cut a long story short, she can't get over my DF. Or, more to the point, the thought of what their relationship was. Rose tinted glasses and all.
I really want to help her move on, once and for all. She refuses to go for counselling, but I wonder if I could get her to read a book, if anyone has any suggestions.
Thanks in advance for any help. I just want her to be happy