Honestly, with a child I would give them a book on a topic I think they would enjoy. If an adult said "suggest a book I might like", I'd also give them somethin

Honestly, with a child I would give them a book on a topic I think they would enjoy. If an adult said "suggest a book I might like", I'd also give them something on a topic I thought was interesting. Saying you want a "serious maths book" really is just too large a range. I could suggest an excellent book on computational group theory (my prefered area of maths), but honestly for most people that wouldn't be something they'd want to read.