I am yet another admirer of Story of O. A less well-known but very well-written novel with a strong S&M theme, which is well worth a look, is Something Leather by Alaistair Gray. And an oddity, but also good in its way, is True Romance by Helen Zahavi.

The stories in BDSM Library which I most enjoy are those with themes of humiliation, modification and mind-control. I prefer non-consensual but not overly violent or snuff stories. And they need to be at least competently written. The most consistently good stories were done by Parker, I think, but there are many other excellent ones on this site. In their various ways, apart from Parker's stories, I have especially enjoyed Casino, Millennium, Paul & Anne, Second Place, Slave to Politics, So Easy, White Slaves, Bra, Welcome to Paradise and Hypno Happiness.