I recently got my hands on the Crooked Moon Source Books and it got me wondering if the Mending spell could be used as a healing spell on the construct races in it.

(And yes, I'm aware the post title is messed up.  Stupid autocorrect.)

No, unless the species in the book have a trait saying otherwise.

Mending can only affect objects, not creatures. Constructs are a creature type.

Some creatures have traits that explicitly say they can regain hit points through the casting of Mending, but that's an explicit exception.

I remember that, for example, the Nimblewright, from Waterdeep: Dragon Heist, has the trait Davyd mentioned.

EDIT: Warmagus1983, you can still update the title.
EDIT2: some slightly related threads:

- How can a Threadborn heal?
- New Healing Spells (Works on anything?) 
- Does healing magic/potions harm undead creatures?