Is the niqabi woman who reads sexual topics to calm her high libido sinful, knowing that she does not masturbate?

Reading books related to marital relations will only exacerbate the problem, not solve it, and is forbidden in your case. Therefore, you must occupy yourself with truth, patience, and chastity until Allah facilitates a righteous spouse for you. Among what helps in maintaining chastity are: frequent fasting, lowering the gaze, closing the doors to temptation, staying away from what incites lust, strengthening one's connection with Allah, accompanying righteous women, occupying one's time with beneficial deeds, and persistent supplication.