My book almost doubled when I moved in with my husband (my books + his books), so I've still got to get through all of his books (minus the ones that we've both read). Charity shops are also a great place to find books.
If you're asking how I choose which books to read, I just look for interesting titles/blurbs or authors that I like, then read a few pages and see if the book seems interesting. I don't like to make decisions based on other people's opinions.