start them off with something like the CBC Nature Of Things Episode on Adult ADHD next have them at least flip through 'Taking Charge Of Adult ADHD' by Barkley
The book, 'Is It You, Me Or Adult ADHD' by Cera is entirely about this issue. It's a book for the partner, not the patient, though patients could read short sections. The book has strategies for reaching the patient and persuading them to seek assessment and to get treatment.
Motivating someone to seek treatment is largely about what is it costing them to not treat the adhd, and to a lesser extent, how hard it is on their loved ones to put up with the symptoms. Even getting them to wander through this site might be enough to motivate. Typically pressure from the partner to get help is not well received, even when it's well meaning.
It's common for adults to first recognize they are adhd when they witness the assessment of their child or partner - so you sure want them to be there.
Leaving a book like 'Taking Charge Of Adult ADHD' in the bathroom can be a great idea. Or have them watch the CBC Nature Of Things episode on adult ADHD with you - see resources for the link.
Sometimes, people won't be willing to do it for their partner, but they might for their kids, and they might be open to suggestion from just about anyone but their partner - so a friend or other family member could make a suggestion.