There are many meanings in the title “touching the void”. Some people may accept the literal meaning of this, as void means empty space. People may believe that as void means emptiness, it suggests that the characters in the book touch emptiness. But what is emptiness.
Emptiness could mean death. If you replace then word void in the title with death the title would be “touching death”. This could be one if the meanings of the title “touching the void” because the characters in the book could be having a near death experience, or might actually die.
Another meaning of void can be space. If you replace the word void in the title with space it means a different thing. If a book was called “touching the space” and you know that the book is about mountain climbers, you could conclude that the book is about climbers that fall down a place where no one has been. This could be another meaning of the book, but different people might have different beliefs about what the book could be about.
In the book joe says that climbing is like a vicious circle. By this he means he means its like an addiction, when he achieves something, he wants to try something even harder. He says “it was a vicious circle” meaning he will kill himself climbing and he knows it. This could be another meaning of void. It could mean addiction. An addiction that will once get you killed as it will make you try something harder and more dangerous. In joes case, the something dangerous is climbing higher mountains.