letadverb

let alone do something | American Dictionary

let alone do something

idiom
and to an even greater degree do something:
Brian would never even read a newspaper, let alone a book.

Note:

Used to emphasize the extreme nature of an action.

let someone/something alone

idiom(alsolet someone/something be)
to not annoy or interrupt someone:
Just let her alone – she’s not bothering you.