Former/failed/flamed-out computational chemist right here. I can't give a recommendation, because I forgot which book I used way back when, but the answer depends somewhat on whether you're interested in "large" or "small" structures. Each will use different techniques.

Molecular mechanics, which is interested in simulation of big biomolecules and the like, ignore quantum mechanical effects, or use a hybrid of quantum/classical mechanics, because the structures of interest are large enough the QM effects can be approximated away.

Other subfields of computational chemistry use only QM, with some approximations to make the programs tractable.