When somebody says serious-but-friendly, I think of:
http://ernestinefont.com/

hhp

Oh yes, I looked at that too. What sans would you suggest, or go it solo?

I don't know if Ernestine would work for body text in a textbook... And I wouldn't use a sans for a textbook either. So maybe find a serif font for the body that echoes something from Ernestine (used for headings); I think it would have to have a rather neutral character (especially if it's a slab too).

hhp

Yes, I see that - but isn’t Ernestine a slab? If not, why not? If so, why would Ernestine not work as body text but Caecilla works fine?

Anyway, the plan is to use sans for headings and inline boxes and so on, then main body as a slab or serif.

Thanks for your thoughts hrant.

I like Karmina Sans with Skolar.
http://www.fontshop.com/fonts/downloads/typetogether/karmina_sans_bundle...

Caecilla isn't as wide and most of all is more neutral. But personally I'm not a fan of it for long text either (and at least Ernestine's slabs are angled, so more comfortable).

But I wouldn't discount a "gentle slab" like Ernestine for headings.

hhp