If you're looking for manga similar to Nozoki Ana, you might like these titles. All recommendations are made by Anime-Planet users like you!
They both cut very deeply in the ecchi/mature theme genre but they are well made or better said well written. It's been so much fun reading them they cover a romance-lovestory in a kind of "hentai" way but still got coverd the story... just check it out and you know it certainly isn't something for everyone but I'm sure there are people out there which will like a good laughter and a pretty unique story handling.
Check them out they are worthwhile.
Both manga have a similarity, main male character dragged into some weird relationship by force. While in Nozoki Ana main female character forced the main lead to peep on his sexual life by blackmailing him, on the other hand in Velvet Kiss the step mother of the female lead character who pulled the strings to be friend with her daughter only to achieve her greedy objective. But in both story both male and female leads fall in love with eachother eventually. [and both contains sexual scenes.]