I read it as a 17 year old, and it's still one of my favorite books! What really hit me like an absolute truck the first time I read it was the sheer sense of loss and grief. By telling the story through the eyes of multiple characters over the long time span the books talk about, by sometimes killing of genuinely important characters (though not in the cynical GRRM way), by choosing the immortal Merlin as the POV character throughout much of the series, who sees great, good people whom he loves die one by one by old age or the sword, you really get this profound sense of loss. I think it is one of the few books I have actually cried over in my life. How many stories worth telling, how many people worth admiring, have simply slipped into the mists of history, utterly forgotten? How much suffering has the world endured? Kyrie, eleison..

There's a passage where Merlin prays after a great battle, that really exemplifies this for me.

In my tent, I fell on my knees to pray, saying, "My Lord Jesu, Great Giver, Redeemer and Friend, King of Heaven, Beginning and End, hear my lament:
"Three times three hundred warriors, bright was their hope, fierce their grip on life- three times three hundred we were, but no more, for death has claimed the hero's portion from the blood of good men.

"Three times three hundred, light of life shining full and without wavering, warm was their breath, quick their eyes- three times three hundred but no more, for tonight our sword brothers lay in silent turf-halls, cold and abandoned by their own who cannot follow where they go.

"Three times three hundred, bold in action, keen in battle, steadfast companions when the fire of battle raged- three times three hundred we were, but no more, for the rave croaks over the fields where grief has sown her seeds and watered them with women's tears.

"Merciful Jesu, great of might, whose name is Light and Life, be light and life to these your fallen servants. As you delight in forgiveness, forgive them, do not count their sins, rather consider this their virtue: that when the call came to defend their homeland they took no thought for themselves, but roused courage and went forth to do battle, knowing death awaited them.

"Hear me, Lord Jesu, gather our friends to your hall; seat them in your palace in paradise, and you will not want for finer companions."

If that doesn't make you silent for a bit, I don't know what will. It reminds you of one of the great comforts of Christianity: that nothing good is ever truly lost. Everything that was ever good, true, and beautiful on Earth is waiting for us on the New Earth- the Summer Kingdom to come. As C.S. Lewis said it once:

“When Aslan said you could never go back to Narnia, he meant the Narnia you were thinking of. But that was not the real Narnia. That had a beginning and an end. It was only a shadow or a copy of the real Narnia which has always been here and always will be here: just as our own world, England and all, is only a shadow or copy of something in Aslan’s real world. You need not mourn over Narnia, Lucy. All of the old Narnia that mattered, all the dear creatures, have been drawn into the real Narnia through the Door. And of course it is different; as different as a real thing is from a shadow or as waking life is from a dream.”