The Bible for sure is #1
As an atheist, I am curious what in the Bible has influenced you. I read it and I found that without the faith part (which makes it a book important to you, no matter if you like it or not) it reads as a story like others. I am not trying to be critical in any way, it is that I failed to find really deep thoughts inside, which would make me think after reading it. It is full of stories which have a moral, not differe…
In fact, there's quite a lot of practical and interesting insights into relationship, finances, discipline and more, for example the book of Proverbs is full of literal advice on dealing with family, children, marriage, business ethics. The letters of Paul to the Romans and Corinthians contain very interesting thought provoking statements on love, relationship, respect. Even if you remove faith completely from the picture you can definitely find very interesting content.