I recommend you start from menga, the real books is still too hard for you, actually we do have many free books online.But the books you need will not be included. However, mengas in your average are free.I will recommend you one most popular one. oeuvre from“几米”，his books are all in deep thought, the most famous one is “向左走，向右走” and “地下铁”，they all have a movie version now.and you can find very fantastic songs in the movie（遇见）. Hope you will like it.you can get books from this website. http://vdisk.weibo.com/wap/?skiplogin=1

the best way to work towards HSK-5 is memorizing HSK-5 wordslist (1,200 words)

Hmm, I don't know too many, but you can try http://chinesereadingpractice.com, it has authentic content. Also, there are browser apps that automatically look up words by mousing over the word. That might help with new articles or other content which is difficult for you.

guys ignore my awkward answer please,I misunderstood the meaning of 'authentic Chinese reading materials',sorry for that

chinasmack.com has articles from popular news stories in China. They're displayed in English, but when you mouse-over the text it displays the Chinese. Voice of America and BBC both have respectable Chinese news coverage. From my experience, it's usually a little easier to find short articles at the VOA site. BBC tends to be a little more in-depth. if you have an iPhone, you can download an app called "Amanda." It delivers 8 short (one two two paragraph) articles. You can alternate between Chinese or English,and it will read them aloud to you. Typically, the most of the articles on the "front page" of the XinHua (http://www.xinhuanet.com/) are only a paragraph or two long. Another app you can try is Pleco. If you have an iPhone you have to pay for some of the features. On android most of it is free. This app has a function that allows you to copy and paste entire articles for reading assistance. http://www.popupchinese.com/lessons/short-stories