Do you recommend looking into any specific more complicated theory or guidance into meditation, or do you think it's sufficient to focus on breathing while letting thoughts flow and be?

Each person's causes and conditions are different, so each bit of guidance has to be tailored to them. That said, I have found great support in Shambhala International. It's a Western Buddhist tradition grounded in the Vajrayana from Tibet. You can find a Center here: http://shambhala.org/centres/find-shambhala-centre/

One excellent text that comes from one of the parent lineages of Shambhala is the Tibetan Book of the Dead, which is basically a preparation for the Bardo, or the "in-between" stages after life. You can read more here: https://en.wikipedia.org/wiki/Bardo_Thodol

There is a vast library of teachings from our senior teachers here: http://shambhala.org/teachings-library/ Plus a bunch of recorded talks by our very controversial founder: https://www.youtube.com/results?search_query=chogyam+trungpa

For Tibetans, the practice of meditation is simply the practice of preparing for death. You are becoming familiar with your mind so you will be able to see through the delusions that arise when your mindstream is no longer attached to a physical body.

Of course, you don't necessarily have to believe in rebirth to find value in these traditions. I have found the community aspects of Shambhala to be quite supportive, even when I had doubts, which in this tradition are expected to be plentiful, and are celebrated.

Happy to answer any questions!

This is a lot to go through, but I've bookmarked your comment so I can take my time. Thanks for sharing.

If you want just the science and the meditation without any mumbo-jumbo, I'd recommend Sam Harris' Waking Up. http://www.amazon.com/Waking-Up-Spirituality-Without-Religio...