The books by Eckhart Tolle are simply inspirational. He sums it up very precisely:
Life takes place in the present. Our identification with the ego and our pain from the past
are the origin of all problems.
'The Work' by Byron Katie is a system to reassess your own convictions and stories.
At first glance it appears very simple. However, once you get into the subject,
you quickly realize that it is very effective.