The Good Book is A. C Grayling's 'Secular Bible'. Much of it is a compendium of ancient writings from around the world. I suspect this bit is written by himself.
The Good CH II vv i, iii-iv, vii-ix
It is in the death of others that our deepest grief and greatest loss comes.
To live is to have a contract with loss. The past eludes us, and carries away what we valued;
some of those we love will surely die before we do and we will mourn them.
For this we must have courage; necessity is hard, so we must accept what is inevitable and unavoidable, and endure.
But even grief abates, and those we grieve, if they could speak, would tell us that they do not wish us to grieve for ever,
But would wish us to remember the best of them, and to return our thoughts to life and the good. And life is the endeavour of the good.
We honour them most, and cherish the memory of them best by obeying the injunction to live, and to seek the good that endures.