It makes me think that there is no one complete philosophy of life. You read the view of one person, and then you’re ready for another. Two authors come to mind, John Steinbeck and Robert Pirsig.
Steinbeck, influenced by his friend Ed Ricketts, wrote Log from the Sea of Cortez, which is technically the account of a marine biology expedition he and his friend conducted together; but of course it was so much more than that and could be thought of as a transcendent voyage into philosophy, mysticism, and a holistic view of life.
There is my own philosophy: mm-theory.com, which, though not directly one on “how to live”, has left me with some insights on how best to live one’s life (namely, by amassing as wide a variety of experiences as one can), though it probably fails your second criterion: “(2) are relatively easy to understand”.
Stoicism is a complete philosophy which is moreso a way of living. It is easy to understand. Nothing supernatural. Originally “God” was “logos,” i.e. nature, or the logic thereof. But in those days everything had to have a nod to some God. Today, it would be called naturalism, I think. Also, it formed much of the basis of Christianity, but before the marketing plan.
I’ve only known one person who was considered by nearly everyone who knew him well to have a perfect character, to be world-class smart, and to not have a “faith” bone in his body. He was a pure Stoic except for an updated view of determinism (probability).
Now nearing the end of my mental life, I would most like to say I was one too, over any other philosophy, to have led the best life. But it’s too late for me.
There are updated books, but I don’t have a title. I think googling and reading reviews is a reliable way to choose books. Wiki has a nice introduction.
I would like to learn more about Stoicism, or if there are similar philosophies.