Does a monsters horror affect an investigator if it's in range ie.. 3 spaces and does not have line of sight of an investigator?

Also does within range work around corners and such for investigators for the purposes of spells and weapons?

There isn't line of sight in the game.

neosmagus is correct. As long as the rules for "within range" are followed (i.e. not through doors, within 3 spaces, etc.) then yes, it works around corners.

LOS never comes into play.