[His eyes, as he stared through the blades of grass, were fierce slits.]

as he stared through the blades of grass の as は〜している時

[It was a naked girl, with her back to him.]
説明不要

[She was not quite naked.]
説明不要

[She wore a broad leather belt round her waist with a hunting knife
in a leather sheath at her right hip.]
説明不要

[The belt made her nakedness extraordinarily erotic.]
make something 〜　の意味は somethingを 〜にする。
>>640>>641>>642>>643>>645>>646>>647

[She stood not more than five yards away on the tideline looking down at
something in her hand.]

looking down at something in her hand の looking は
【分詞構文】(http://choidebu.com/bunpou/bunshione.htm)。
彼女の手のひらにある何かを見下ろし【ながら】