pg 37-38 and 108-109
Honestly I don't find the cover rules as written all that big a deal particularly since true line of sight is still a factor, complaining that a unit in the open behind the hard cover of ruins but not actually within the ruins themselves gets one less on their cover save for lack of something else to hide behind seems silly, I personally find the 6+(4+ if you go to ground) cover save of craters to be more objectionable when most other forms of cover gives at least a 5+ including intervening models
Maybe its just me but I find it hard to stomach Gretchin giving you better cover than earth and rock.